Welcome to the forum and unicycle world.
So you want to upgrade from a 20"
Just a few thoughts from my experience before “deciding” and shopping.
I would highly recommend that you network/find other unicyclists and borrow/try a 24" and 26/27.5 before you spend $500+. Try it before you buy it
a.) Bigger wheel size and crank length combinations are going to be a real adjustment moving up from a 20". When I went from 20" & 125mm cranks to a 24"/150mm…there was a few weeks where I felt like a beginner, again. Took me about 6 months before I felt more confident on the bigger unicycle. I even bought replacement cranks to 125mm during this uncomfortable stage. It takes more overall physical manipulation to control a bigger/heavier unicycle. Adjusting to the feel and timing of a longer crank is a skill to master. Also the few extra height = more painful ankle sprain if you fall wrong way.
b.) Learning basic tricks like free mounting, drops, bunny hopping and idling was hard. I had to go back to the smaller unicycle. A bigger wheel was more fun to ride…but “can’t learn new tricks on it”. However, I have actually learned backwards and do SIF tricks on my 24", but I cannot imagine being able to do this on the next larger size.
c.) Maneuverability was not easy. The same thing with learning new tricks, issue. Doing tight turns while trying to balance over rocks and bumps, even harder.
d.) Falling down. Yup, just because you can ride a 20" on a flat street won’t help you off road. It’s a new skill, again. Expect to fall down a lot. From a 26" the force of impact is compounded not including twisting and flipping as you fall.
I don’t mean to scare or discourage your progress(but falling down is scary and adjusting to new unicycles is not instant). There’s a reason why so many unicyclists just upgrade from 20" to 24", instead of quantum leap to 20" to 27.5" or 36". However, if you have the skill level to do so…go for it. I’m sure those “experts” who have done this, will be writing back with their opinions.