A good question, I can relate to your problem with the seatback. However, there actually aren't strollers with a backrest longer than that. What you need in this case it to look at strollers that have enough space TO THE CANOPY as you won't find longer bakcrests anymore. That means non-reversible pushchairs for sure - no reversible one will have enough space for a tall child like yours anymore.
I personally would check the Britax B-Motion 4 Plus as this is a very spacious model good for tall toddlers (even if its seat back is NOT too upright, unfortunately). Another very roomy option would be the Valco Trend - it has its flaws, as every buggy does, but the seat space is very generous indeed.
Next in line, the Oyster Zero is very wide AND with ample space to the canopy, and similar mid-size lightweight buggies will also do the trick (Baby Jogger City Mini GT2 or the Baby Jogger City Mini2 would be my choices if the budget allowed it; and look at the Thule Spring as well).