I had everything you say, solved it with a rehab programme, a slight lowering of saddle with more setback. I have a minor physical leg length discrepancy and scoliosis (side curve spine) resulting in left shoulder sitting noticeable further forward - diagnosed by a physio and confirmed by consultant. Manifested on bike predominantly as lower back pain.
I am very wary of chiropractors, have seen people pay for years of ‘treatment’ which does not work to eradicate the issue but instead treats symptoms. Great business model but not the best medical approach.
That photo looks to be taken at an angle (see edge of bed), if it was taken barefoot perpendicular to the bed with a set protocol to ensure you were laying evenly (which I have had a physio / bike fitter do) it might look very different. The cynic in me would ask if they have used an image to sell you a service.
A leg length discrepancy would IMHO, unless severe - in which case permanent corrective orthotics are likely necessary for all activities, only likely to make you sit off centre if the saddle is too high or extremely low, I would try dropping the saddle further in the first instance.
You don’t mention rest of fit but it could be out of, or to the extremes of, your tolerances, or bike setup out of alignment (bars off or not straight). Also the saddle could be set out of position for you or simply not suitable.
All of that would contribute too to ‘uneven peak power phase’, the 5% isn’t something that would worry me at all, neither 48/54 split.
The lower back working hard usually points to key fit issues or suggests weight too far forwards (usually then caused by reach too far or low, or insufficient saddle setback).
Spinal curvature is not something a figure 4 stretch is going to sort, that’s usually for hip flexors.
These are the sorts of exercises (plus more and advanced versions) I was set to do in this case:
Also deadbugs for core, glute bridges (next step on from pelvic tilt - first 2 legs then onto one), and plank sets (front then side, starting short position and over time working up to long, then again over time working to planks with destabilisation).
I hope you have found the answer but personally I would be taking another path.