No hard feelings Winding!
I understand the āweaknessā aspect now, thanks for the details.
Have you looked into the āActivating Your Horseās Coreā book? Itās pricey for what it is but itās got really good in-hand exercises which (obviously) engage the core, but more than that, engage that flexion of the back end. Sort of like āstatic flexionā - flex/lift and hold. Also, if you havenāt incorporated backing, thatās really useful for strengthening the SI area (again assuming just weak, not damage). My TB mare tends to be hyperflexible there, and when I was doing regular backing in-hand with her, the improvement was immense in fairly short order
Also - if you donāt have one, a BOT mesh sheet might help, even if itās only on for a couple hours a day, preferably taken off right before heās worked.