Hey, Katya,
I know youmention it's not about butt to canopy that much, but if the canopy will be too close to the end of the backrest, it will be too small too soon anyhow. Thus, I'd definitely go for a jogger which are more robust and overall roomier, so, a BOB perhaps, those have tall seatbacks. I personally really like the space of the Baby Jogger City Elite, and I know moms use that one even for bigger special needs kids, so perhaps worth trying out... There won't be many more that are even more spacious, honestly.
My strongest suggestion here would, however, go and try it IRL. Cause each child is different and the way they're sitting and how their body is in a stroller seriously makes a difference, and so trying the stroller before buying, especially with such a need, is, I would say, the most important here.