For legs squats will load the spine better than any other lift resulting in more adaptation rep for rep than most other lifts. The problem can be your range of motion and mobility. If you cannot get a full range of motion then you may want to look to other methods. Single dumbbell squats, Bulgarian split squats, machine squats, hack squats, front squats etc. for rep range I have always seen more hypertrophy from a higher rep range. Sets of 15 to 20 absolutely cook my legs, but I always keep sets of 6-10 in the mix as well. If I wanted more strength I would go even lower, 1-5 reps. A lot of the time I do 5-6 for a more power building approach. Really just depends and I like to mix it up. But my overall goals are hypertrophy and I am not currently competing in anything.
As far as how many sessions a week... that depends on your recovery ability, but maybe going through a short phase of higher leg frequency would be beneficial. Maybe a 2-4 week block when you add in another leg day. One day can be heavier loads, one day more moderate with higher rep ranges. Also you can split the sessions into a quad dominate workout and a hamstring dominate one.