These Bulgarian split squats are so hard to pull off. I barely did 2 sets of 4 just bodyweight. And with awful form...
How are you setting them up? There’s a certain amount of individuality when it comes to the height and distance of where you can plant your non-active* foot in such a way as to be comfortable and efficient, e.g. a standard bench may be too high for some people. My preference has always been to use a racked barbell or the Smith machine bar and a squat pad (one of the few actual uses for it) or rolled mat to provide some cushioning since you can adjust the height appropriately.
If balance is an issue then buy a wooden broomstick handle (also a good random thing to have in a home gym for doing shoulder dislocations and some other mobility work) and use that as a sort of walking stick-like aid.
*The rear leg will never be totally inactive in any split squat variation, but the goal should be to minimise how much it helps drive you back up - it should just be there for stability - which obviously makes it harder but also means you won’t have to use as much weight.