Keep working on that motor control and bracing sequence.  However, if you are missing some active flexion, your back will round to give slack.

What else?  Smash high hammies with a ball, voodoo floss the groin area while working on flexion, and I’d look upstream in the t-spine to give that lumbar some slack, and check out that anterior chain smashing psoas, quads, then couch stretch the crap out it.