There are plenty of sissy squat variations. Look for videos on Vince Gironda’s sissy squats. You could also do variations with a dumbbell or 2 dumbbells, in a goblet squat/front squat
position, or with a weighted vest.
You could experiment with a smith machine front squat, if you have access.
Because of balance and being at a mechanical disadvantage you probably won’t need much weight to get a good workout.
There are sissy squat machines(not really a machine because there are no moving parts, actually a Roman chair, but still great for quads)
If you check out old photo’s of Tom Platz, you will see him doing 2 sissy squat variations, up on his toes,(and also with the hips off the back rest ) on a hack squat machine.
You are going to want to go real light or have seriously bulletproof knees.