I think it builds in difficulty each step up in distance, because the techniques to get farther build on one another. Thus, the hardest in my opinion would be going from 450 to 500ft. I've been playing for 10 years off and on. For me the progression has gone something like this:
Getting to the 200-250' range comes with learning how to properly hold and release a disc level. Early in my disc golf career, once I learned to release nose-down and come across my body, this is the distance I could get and took about a month or two of playing.
Moving up to 300' was learning to control that level release whilst trying to put more power into the throw and incorporating a run-up/x-step. The run-up/x-step wasn't flawless, but just getting that momentum going forward was enough to generate the extra snap to get the disc to fly. This took several months of playing before I got comfortable at this distance
Jumping to 350' for me was getting comfortable and perfecting the run-up. For me, finding a routine that felt the same every time was key. This came between 1-2 years of playing for me.
The transition from 350' to 400' for me was learning to generate more torque with my body while maintaining control. The most difficult in my experience was learning to not "muscle up" too much when trying to throw the disc faster. When you do this, your body tenses up and it actually inhibits the faster movement you're looking for. Learning to stay smooth and fluid was the biggest key. 400' came to me after about 3-4 years of regularly playing, but took about 4-5 years before I could hit it consistently.
I think the key moving from 400-450' was adding controlled snap to my arm and wrist when throwing, on top of smooth form. This adds a bit of extra torque to the disc enabling the further flight. In my experience, when I started to consciously think about snapping wrist at the end of the throw is when I started seeing the gains to 450'. This was difficult to learn to do consistently without turning a disc over too much or grip locking it. I wasn't able to consistently hit 450' until I had been playing for 7 years or so. 450' is my maximum comfortable range I throw with relative accuracy, i.e. throwing on a relative low and straight line, with little to not run and subtle fade at the end (For me, my go-to is an MVP Photon for these shots).
Getting from 450' to 500' I feel is the most difficult, because it requires all of the form technique above, but really perfecting it all into one smooth motion. The smallest flaws in your form will keep you from getting there. I can hit 500' but it requires some flex on my shots and it's not something I'll comfortably try to throw on the course unless it's a wide open field without barriers and OB.
500'+ is when you put perfect form together with less stable discs and really air them out on huge flex lines. I think even at the highest level this is quite variable. It takes just the right angle of release to hit a huge distance line, and it's very easy to angle it to low and hyzer out to soon, or angle up to much and flip the disc over.