I just made the move from a 24” muni to a 36”er a few months ago. More specifically, I went from a 24” Qu-ax with 170mm cranks to the Nimbus Impulse with 125mm cranks. I’m also about 5’9”, so I think I can relate to your situation pretty well.
As for being tall enough, you should be fine. I cut a couple inches off my seat post to get the seat low enough to ride it and also leave some room for adjustments either way. Just measure your inseam and check the unicycle.com website for figuring out the correct seat post length.
I’m probably only an intermediate rider, but adjusting was fairly easy for me. The biggest hurdle was adjusting to the extra force needed to get the wheel going, but I was able to get going after a couple of tries using a wall to mount. I was able to freemount for the first time on it after about a 6 attempts, and had about a 20%-50% success rate for the first few days of riding. Slowing down and turning also took some getting used to with the larger wheel and shorter cranks. Overall, I started feeling comfortable on it within an hour and more comfortable the more I rode of course. Since you already know how to ride, I think you should be able to adjust fairly quickly.
I’ve never ridden a 29” so I can’t comment on that, but I doubt it’d be too much harder to switch to a 36” than a 29”. Personally, I’m very happy with the 36”. If I want to do distance riding then the 36” allows me to do that easily with greater speed, and if I want to go off road then my muni is still there for that. I don’t feel the need for something in between right now. However, if I had a Schlumpf hub then I might change my mind on that. So if money isn’t that important then you may want to look into those. A lot of people who have one will put it on a 26” or 29” to have a single good uni that’s good for both muni and road. One advantage to a 29” though is its size. A 36” can take up a lot of space, so depending on your needs, a 29” may be better for you for traveling around with, whether on a car, plane, bus, etc.
You didn’t ask about crank length or brakes, both of which tend to be discussed often on this site and tend to be more of personal preference issues. As for myself, 125 cranks have worked well for me. They seem to offer good speed and have enough length to get me up most hills and maneuver without much problem. I also like having a brake since it saves wear on my knees going down steep hills and allows me to stop more quickly.
You should be tall enough to use the 36" provided you don’t have extremely short legs. I’m 5’6" with a 29" inseam and have the seat all the way down on my KH 36. I can use the 150mm pedals at this setting but the 125’s require a bit less leg stretch.
There will be a learning curve. The 36" can be tough to free mount and is a bit of a beast to get moving. Once moving it is a very fast, powerfull ride that can be somewhat intimidating until you get used to it. It is also reasonably hard to manouver at slower speeds and in heavier traffic.
I have not rode a 29" but am thinking that I might be happier with one of those than my 36, based on the in city riding that I normally do.
Try to get on both of these units prior to purchase to give youself an idea of what might be best for you.
I’m a big fan of KH unicycles so would recomend you go that route especially for the 36 if cash isn’t an issue.