Hogar, elevated dumbbell split squats( hold one dumbbell with one hand , hold something to support other hand) seem to work well for a bad back. I don’t use any squats that are shoulder loaded or dead lifts anymore.
Could try unilateral work: step up to a bench of suitable height, with added weight via vest or dumbbells; split stance squat, such as Rear Foot Elevated Split Squat (a.k.a. Bulgarian Split Squat). T-Nation has several articles on unilateral leg work.
Belt squats are another possibility. Easier if you have access to a machine, but there are cheaper ways to jury rig something, depending on the equipment you have. Google “belt squats” and you will find a variety of DIY solutions.
Try wall squats