Interesting thread. I have been diagnosed with osteo arthritis in my left SI joint, adding to my laundry list of middle age (or maybe it's elderly now) maladies, up to and including a L5-S1 fusion. However, I find that riding either full upright, drop bars, or bents provide the best relief from pain than anything. Although I am now taking Diclofenac and CBD oil I have determined that by far the best practice is to build your core muscles. Abs, abs, and more abs. I'm sure I have pretty well defined abs under all the flab. When bending, lifting, or generally doing what I am not supposed to and it starts to hurt back there, I realize I am using poor posture and when I stand up straight and suck that gut back in the pain almost magically disappears. I also learned early on to tell the difference between SI pain and lower back pain. This may or may not be useful to the OP, just my experience.