Just another thought. If your scans showed no obvious blockage, may your sphincter be cramping because you have to tense it often to hold the pressure from gas inside the bowel? Then you would have to work in the opposite direction, i.e. try to relax your pelvic floor by breathing deeply into your stomach when lying (when you put your hand on your stomach it should move up when breating in). Or you can try to tense the sphincter / pelvic floor really strong for 10 seconds on the toilet, like trying to hold your stool back. That helps to relax that muscles completely afterwards when you release them (the principle of progressive muscle relaxation).
A warm sitz bath may also help for relaxing.
You wrote that you have a very strict and healthy diet. Especially brown rice is said to have a stuffing effect. How many BM do you have per day (approx., I know it's difficult to tell when you can't have regular BM on the toilet) and is your output more solid or liquid? Have you tried psyllium husk for regulating stool consistency?