Seems like you’ve already tested symptoms for diarrhea, constipation, hemorrhagic gastroenteritis, anal sac inflammation, enlarged prostate, and gastrointestinal obstruction (all causes painful defecation). While looking at blood test and x-rays, it’s better to check these off the list as barium testing or invasive surgery should be the absolute last resort.
An anal gland may be ruptured or has a tumor.
In the meantime, switch to feeding only wet food. I recommend ZiwiPeak, Lotus or Weruva brands. Flavor the water bowls and offer bowls of broth, anything to increase moisture intake.
