One should never draw too far reaching conclusions about throws performed on wet or soggy ground so my comments are like you had thrown on solid gorund where your right heel would not dig into the ground. Because that is a different condition and that would represent normal technique which is what one should improve.
How fast were you running with that throw? If i had to guess it was a normal golf x step where faster steps and a run up would add power and difficulty of maintaining form.
You had a spin out with the final step so your right foot never stopped or really slowed down in the pivot after the heel touched the ground. That makes twisting the hips and turning the shoulders right harder than having the right leg jammed in placve for a moment starting at the heel first touching the ground. Getting the elbow forward is also harder but in your case not knowing why it was sore i do not know if adding more snap and thus stress on the area is a good idea. At least for now. It is normal for players your age to throw so much that tendon inflammations occur because the muscles and tendons are not necessarily as tough as with older people even though they may be as large depending on how much you work out. Have you had your arm checked out for what was wrong? Tendon damage can take half a year with no stress to heal. Now is too early to train outside even for young people if there are underlying injuries. You want to give the arm time to heal in time for the summer. You will have time before Konopiste Open.
Your final step was about 10-13 cm too long for that speed. The rear leg was fairly straight and your body upright when the disc left so the leg power generation is actually not that bad. The body turning right in the end of the throw adding to the elbow area tendon plyometric loading thus snap can be significantly helped with shortening the final step so that the right should is above the right heel when the disc rips or slightly ahead if you go for a head high or lower throw. The final step length increases with running speed so the momentum will not push you too far beyond the heel with the shoulder endangering falling on your face so a faster ran start to the throw has the final step in the corect range now as long as the right legf does not collapse so that the body can shift the weight forward to shoulder above heel.
The lecver length of the arm is maximal when the throwing arm is at a 90 degree angle to the body. It is a minor tweak and keeping the arm at 90 relative to the body is more of an accuracy and consistency issue. Your reach back to release with the arm was a hyzer but the way you planted the right leg to the left of the left leg tilts the legs to the right toward anhyzer. The torso was also tilted hyzer so you bent forward at the hips. So you have different parts of the body tilted at different directions meaning that when you tire some place will tire out faster than others if you have muscle power imbalances and many do. The chain is as weak as the weakest link and if some place gives out that place will not necessarily be in the angle you are used to which messes with the hyzer angles. Not being upright has the disc leaving lower eating a meter or few of distance but there is also a power loss from more than possible body weight being away from hte axis of rotation of the heel pivot. Think of ice skaters doing a pirouette rotating slowly when the arms, weight, being away from the skates and rotating faster with the arms tucked in. It is the same phenomenon but in your case early on i am not sure if it is healthy to add rotational speed thus more plyometric extension to the arm not knowing how hurt you are and what the cause of the aches are. So this might be more of a long term tweak.
The next peice is dangerous to even long trained very muscular atheletes and even worse for non top athletes and even worse for juniors from a helath perspective. Hip problems have occured to at least one hall of famer and and one open silver medalist i assume from the way he warms up and acts during rounds. The way with which they have more power to the rotation of the body is to maximize the efficiency of the kinetic chain by passing the energy up from the legs to the twisting of the hips and upwards by jamming the foot well in place. So much so that the heel pivot may only really start after the disc has ripped out. The problem with this super hard jamming is that everyhing from the toes to the neck takes a jolt at the heel landing where Olympic javelin thrower Tero Pitkämäki (FIN) has had his in place jammed final step plant measured at IIRC over 1000 kg equivalent pressure. That is like standin on one leg with a small car on you. So you can imagine how hard everything is stressed and and how forcefully the femur is being jammed up to the hip socket. There is a small muscle or a tendon in the hip socket that my elementary school biology teacher said gets damaged by spagats and such in ballet dancers making many of them retire at 40. Now imagine somebody kicking you in the knee inline with the femur puishing it in to the hip socket on every drive. Sounds dangerous for the soft tissus inside the hip socket. Those ballet dancers in worst case scenarios have a hard time walking normally. So tomas Ekström has changed his style from when he was young as a result. Markus Källström seems to have taken notice judging from how large his lower body and leg muscles. I wish i remembered where i saw him show him lifting weights so that i couls see how much he trained with. It is a lot. And still you see him kick his right knee to the left and up during rounds hopefully helping him in the hip area.
You can jam up the hip also by running from right to left well diagonally across the tee pad like 30+ degrees. The right leg cannot twist forever to the left from the hip socket so eventually the hip will jam in place forcing the momentum in the kinetic shain move up to twist the torso right at the hips. So the efficiency of energy transfer is maximized when as little as possible is wasted on pivoting on the ground (you are not) and the energy needs to go somewhere so it takes the path of least resistancy just like rivers and electricity twisting the torso right. The downside is the same as in the previous paragraph but you stress a different part of the hip i assume. I am not medically trained so i cannot say for sure and whether one way is safer than the other.
Unfortunately not jamming up hard until the rip or running straight at the target both reduce generated power because there are more energy losses in the kinetic chain moving the energy from the legs to the torso twisting but long term health benefits may be worth it. There are diagonal runners throwing past 170 m with fairly little leg speed which is not as bad as running like a 100 m sprint then jamming up the leg in place if the direction of the twist does not make more damage than in a straight line throw.
There is another disadvantage to diagonal running in that it is excessivley hard to see the target for visual aiming per the tips of Clime, which are eyes to the right corner. Possibly also turning the ahed of the torso a bit very late. Those work only from mild hyzers to annies where the neck vertebrae support the weight of the head. It will not work in steep hyzers like a golf swing where the head weight will mess up with the hyzer angle. Annies are easier in steeper angles too because the neck muscles support the head weight better than in hyzers. Not aiming visually is leaving you with one less tool. IME utilizing concurrent aiming methods helps me hit gaps better.
There is one form that does not jam the leg and bang up your body as hard as jamming the leg in place until or very close to the rip or diagonal running but still produces more power than regular straight line moderate short jamming of the leg. The double pivot suffers from such a quick= powerful turning that visual aiming is hard to nigh impossible but it need not plant as hard as possible to gain a distance benefit over a straight line run. The double pivot is ran in an arc running somewhat diagonally to the left in the first two steps of the x step and planting to the center of the tee after the x step has pivoted on the left ball of the foot. The double pivot increases the reach back length thus the time available for acceleration and also weight shift distance and rotational momentum just like a 360 throw. Just not in the same scale but much more controllably for practical golf applications. So you have lots of variations to choose from. It all depends on what risks you want to take and which downsides you can can live with. To be able to make an informed decision you need to check if visual aiming helps your accuracy and consistency. It will not be easy at first and you may not get everything out of it at first because as everything it has a learning curve and there are tricks to make it work. Like picking a clearly visible target for height and sideways aiming. Once you've established the results of visual aiming you can decide that is unecessary, necessary or neutral. That leaves only health and life priorities in possibly needing to work out like any top athlete in any sport with the necessary time and effort investment possibly also costs too. Then you can make an informed decision on which form to pick and train. And then you may notice that Scott Stokely is right in recommending a shorter maximum accuracy and consistency form as a basis and a different form for longer than those throws. It is healthier not to jam up the leg fully or jam up the leg to hip turning range induced whatever the heck happens inside the hip socket when you do not need the extra distance. I do not know if there are any really active players who switch forms depending on the situation so i do not know how dangerous it is to use those harder forms at times vs all the time from approaches to drives. I did an approach test on a cramped soccer field because i no room to throw anything powered both straight and diagonal stand stills two weeks ago. I saw no advantage to diagonal and it is not the first time i've tried it in approaches to drives. To be clear i see no benefits to being diagonally positioned in approaches but there is potential in it in drives if you are willing to risk the added stress on your body.
To me a straight line non fully jammed plant step is enough for most cases so i double pivot basically never. And being too banged up as it is for life i do not need to risk my health any more with those more stressing forms despite the distance potential gains. Note that a double pivot can be jammed fully too for even more distance potentials and more health risks most likely vs a straight line fully jammed up throw. YMMV with which form or combinations you choose.
You reach back far for shorter max accuracy throws and too short for max d straight run shots. Your head did not turn enough away from the target for maximum reach back distances but you were only a few cms off but it can mean a couple of meters of lost distance. Depending on the distance for max accuracy throws you may not need to turn the head more than 70 degrees away from the target. So there is a lot of difference to what you did with that throw.
So i'd straighten up your posture and shorten the steps 10-13 cm at that speed and choose the running direction and planting position based on the desired angle of the throw. Flat shots have the x steps land about on the center line of the tee and hyzers come from left to right with the plant landing to the right of the running direction and annies are the reverse.
