I'm not sure a shorter crank arm on the offending side is the answer. No only is the length disparity accentuated, but now the short leg & long leg doing are doing a different amount of work by way of travelling different distances along the circumpherence of the crank rotation. This surely would result is a strength disparity very soon.

I think step one is figure out how each leg was different first. Heel to knee? Knee to hip? Are both feet the same size? Then proceeding accordingly. An insole on one shoe or a shim (or both) would get the legs the same length. Turning the seat a degree or 2 could get some symmetry if your femurs are different. Adjusting cleat position could be the answer for different sized feet or leg length.

I had hip issues. After lots of physical therapy for my psoas tendon. My left hip always gave me trouble but improved. Switching to smaller 170mm cranks solved part of the issue by limiting the size of the pedal circle. Combined with a shock absorbing seat post that I leave the "lock out" nut loose so that it turns left & right a few degrees with every stroke completly solved it. I could tighten it, but it's designed to engage or disengage the shock absorbing function. That it has a few degrees of clockwise/anticlockwise slop is a happy coincidence.

It turns out that an MRI showed a bony protrusion in my hip and the psoas tendon was snapping across it with every pedal stroke. The smaller crankset & different seat post meant it didn't snap back & forth any more.

My guess is (& it's completely a guess) that whatever the issue it is not being helped by a lack of flexability. Cyclists then do have tight hip flexors, short psoas, limited IT band flexability, and short strong glute/hamstrings. All of which make for back pain. Older cyclists just get a shorter stem & ride more upright rather than stretch. Too bad, they could regain lots of speed from their youth if they'd take on a stretching routine.

Good luck