I don't quite get why you think this is your back? It might be worth thinking about your lower back for injury prevention, but this is quite clearly a core strength issue, primarily abs. Leg raises aren't a bad start. Steep bouldering (enough that you can keep your feet on) is obviously a good thing to work on.
getting my feet back on after a cut-loose needs multiple "stabs" with momentum.
Recently I've noticed that my ability to keep my feet on the steepest climbing is pretty poor, and getting my feet back on after a cut-loose needs multiple "stabs" with momentum.
I had thought about lacking accuracy.