It kind of depends what kind of shoulder soreness you're talking about. I assumed you meant standard muscle soreness (usually deltoid for me back when my form was worse and I started playing a lot), but it's more of a concern if there's joint pain like a lot of these guys are mentioning.
Either way, there's probably form stuff at play. I'm probably younger than you are, but not in my twenties anymore. Lately, I've been playing a lot but almost never get sore shoulders anymore, even playing ~90 holes per day for a couple days straight. Part of it is that my throwing muscles are getting stronger, but part of it is an improved form where I'm not "strong arming" as much. You mentioned watching guys throw really far with effortless throws. They're not strong arming at all; they just have a nice smooth swing plane in their arm/shoulders/spine that syncs up great with their core muscles, leg muscles, weight shift, bracing, etc.
Again, there's way too many resources on here if you really want to get into it. Some people post videos of form, or talk about what discs they're throwing and how they fly. It looks like you're also trying to get in touch with local guys who can help. That's always a good idea. Talking about stuff online is never as good as getting out on the course with some better players.
