I’m pretty sure there’s a team in Paraguay who have modified the microscope as you suggest, but I don’t think they are on the forum, the only mention of them was by me. @biodotpe you’ve not tried this, have you?
If you are not getting enough torque out of the 28BYJ-48 motors, is that because you’ve somehow modified the stage? If not, it might indicate something is wrong with the build: they are usually not short of torque, particularly with the default gear ratio.
If you’re looking for more speed, then NEMA steppers would definitely deliver that. NEMA17 will have plenty torque - I did try NEMA11 in the early days, and found that they got so hot they melted the plastic, and even then only just had enough torque.