Hi
I think Alophysio is very wise. The problem does sound like loading, with resultant irritation and tethering. Have you checked leg length? Either real or apparent leg length discrepancy can cause excessive loading through one side of the seat. Apparent leg length discrepancy will be caused by tighter muscles of the erector spinae group on that side, hitching the hip. A weakness on one side will also cause more effort on that side when pushing, with a postural list onto the seat, increasing pressure onto the sciatic nerve. This in turn causes inflammation, tethering, and a greater focus for pressure, leading to potential weakness, perpetuating the cycle ( sorry about that pun).

If there is a leg shortening, address the muscles of the low back via appropriate stretching, and if a real leg length problem have the shoe or cleat / crank adjusted to compensate. Sometimes a number of areas need to be addressed to sort out the various compensatory mechanisms that have developed.

Very deep tissue massage through the Glut Max to piriformis and the tethering are necessary. Trigger point dry needling (if available), can be of assistance. Postural stretching for the shortened side is imperative, as are stretches for the hamstrings and gluteals, but only after deep massage (friction type - to the point of bruising).

Another option is to identify the pressure area and make up a temporary pressure relieving pad for the bike nicks or seat.

Good luck. Let me know the relevance of the above - and any outcome.

Last resort is a cortisone injection - avoid if possible.