What about the slope of your garage floor? Most garages aren't perfectly flat, and that tiny angle can mess with your form without you noticing. My old place had a dip in one corner, and I always felt weird until I checked it with a level. It's wild how such a small thing can lead to back pain after workouts. Maybe see if your floor is level before focusing on stretches?
Levels don't lie and you're right that garages are rarely flat. But let's be real here, a half inch of slope over eight feet isn't gonna wreck your back unless you're doing something really unbalanced like single leg deadlifts on the low side. Most back pain from home gyms comes from people using crappy secondhand bars with no knurling or slamming weights down on concrete. Fix your equipment before you blame the floor.