Hey there, Magi,
The smaller and lighter a stroller is going to get, the simpler and usually less spacious it is going to be, normally. Very few have a tall seatback, unfortunately, but out of those few, I would definitely point out the Bugaboo Butterfly with a 56 cm backrest (it is still a small, narrow-ish pushchair, though, you need to count on that when getting a small buggy), and also the Joolz Aer.
From the reversible models, the Cybex Mios has a 54 cm backrest, which is a great length, however, the canopy is right above that, so even if spacious for a parent-facing option, you will always have it a bit different for a taller toddler, compared to only forward-facing models.
I will also mention the Valco Trend out of the lightweight models, as that one is super roomy overall, and the backrest is 50 cm tall (plus space to the top of the canopy). The Baby Jogger City Mini2 is also a great light option with a 50cm tall seatback, and I would definitely check the ABC Design Avus, too.