When my daughter was in a walking boot last year, we rented a 16-inch chair (at least I'm pretty sure that's what it was) with legs that could be raised from Orlando Medical Rentals. It was pretty much like this one:
https://www.orlandomedicalrentals.com/equipment.asp?action=category&category=10&key=RWC
It was a huge help for her to have her leg elevated for most of the day, so I would definitely look into elevating leg rests.
It also had a tall back with a removable top portion. I'm 5'8" and the handlebars were definitely too low for me, and of course I forgot the handlebar entensions at home (doh!). I found it super easy just to hold on to the poles on the headrest and push her around.
She never complained about a sore butt--but maybe that was because her foot was still in a fair amount of pain, but I can see how a seat cushion would make it a more pleasant experience.
Orlando Medical Rentals was great to work with. They brought a backup chair in case the first one didn't fit--which it did. Pickup and dropoff couldn't have been easier!
Most of the ride lines are totally wheelchair accessible. Many of the loading areas will have an area where you will wait and get out of the chair--just look for signage, because sometimes there's a little gate for wheelchairs before the "regular" entrance to the boarding area. You leave the chair off to the side and either return to it when you're done, or the CMs move it to the exit and it's waiting there for you. Honestly, it's super easy. We frequently just asked the CM at the entrance what to do before entering the line--and often times, they'll just tell you what to do if they see you pushing someone.
I will say that Animal Kingdom was ROUGH in places. Like, literally, the ground is rough, and it makes pushing pretty tiring. Overall, though, it was a lot easier than I worried about, and the chair folded down and fit in the trunk of our rental car.
The first time we were putting the chair together in the lot, a parking attendant scolded us for not asking for the Medical Parking. We said we didn't have a handicapped permit and he said it didn't matter. We didn't even know there was such a thing, but it was definitely a great tip. After that, we just rolled down the window and talked to a parking attendant and explained that our daughter had a wheelchair and asked for medical parking or medical overflow parking. There are spaces in that lot that don't require a handicapped placard, and they are much closer to the entrance. Still a bit of a walk, but much easier than trying to get a folded up wheelchair in the parking tram!