One of the strengths of MMA, and the common feeder styles (wrestling, BJJ, boxing) is the absence of a curriculum. Once you have a set curriculum, you get stuck with having to use it, and then you've got everyone training things they really shouldn't be bothering with, as better methods have been discovered.
Bruce Lee touched on this with the idea of constantly emptying and refilling the cup with water. As you discover something you've been doing isn't useful anymore, you drop it. If you find a better way of doing something, you use that instead.
That's inherrent in MMA - mixing and matching aspects from various styles. As you're introducing new elements, you find some things just don't work. You have to ditch them as a result. You still keep the core though, and the adaptations you've made still work in the original style. With boxing you would stop bobbing and weaving, but you'd still make good use of jabs, crosses and uppercuts, and you'd still work on slipping punches and working combinations. It wouldn't exactly look like orthodox boxing when you box, but it still is boxing nonetheless. With wrestling you'd eliminate going flat to your stomach when someone has your back and would instead heavily emphasise sit-outs. When you wrestle you'd be known for doing a lot of sit-outs, but it would still obviously be wrestling. In BJJ you'd have to stop relying on techniques that use gi grips, and would instead emphasise overhooks and underhooks, some of the more avant garde guards would also be ill-advised. DLR isn't a great idea when punches are allowed. Still, you'd be working closed and open full guards, half guard, butterfly guard, and bread and butter submissions like the armbar and RNC, and when you do BJJ it would still clearly be BJJ.
As a result, you can go to a boxing gym, and assuming the instructor is open minded and doesn't require you to do things only one way, you can tailor your boxing style to be suitable to MMA. If you can find a wrestling club for adults, you can change up your strategy when you wrestle, and work on standing up even if the scoring doesn't encourage that. You can go to a BJJ school and focus on what works for MMA, while still advancing. You won't be a permanent white belt because you don't like using X-Guard or DLR, and you prefer not to bother with Ezekiel chokes.
Try training Karate while refusing to chamber your hand because you want to keep it up, or advancing in Judo if you don't want to do Ippon-Seioi-Nage because it's too risky in MMA. It would have to be a really open minded instructor for that to work.
MMA is all about being the best fighter, and that requires you not to be constrained by a curriculum, so you're not spending time doing anything but whatever helps you the most. You're not going to find many MMA schools with an established curriculum as a result, and the ones you find that do have it probably aren't producing any champions.