I'm a tall guy with poor mobility. I've been at the gym for a year and working on my mobility every workout. I improved massively in many areas except for my ankle dorsiflexion, which still sucks. While I can go low in the squat, it requires me to lean forward and I get sore (DOMS, not pain) lower back after squatting. Recently I started using slant boards and now I can feel the squats in my quads, and my back doesn't get sore anymore. But it strains my knees a lot, even to the point of sharp pain sometimes, which prompted me to back off to very light squats for high reps to rehab. My question is, is low back soreness acceptable, or should I stick to the slant boards and strengthen my quads and knees? Any exercise ideas for ankle mobility? I'm doing goblet squats, squat holds for time, banded ankle dorsiflexion and calf static stretches after workouts.

I can post some videos if required.