You and your veterinarian have done an excellent job in trying to figure this out .

what a dedicated owner you are and what a lucky little birdie to have you.
It's possible this is a chronic very little mini amount of blood being lost in gi , if that's even the case , but if it was the body would easily keep up. I have a chronic bleeding kidney thing and the tiny amounts dint effect my rbc .
The poop pic you shared wasn't crazy abnormal, just a tad off. Normal amount of white urates( definitely getting enough water) ,no excess urine , fecal part still formed .
Nothing is clearly pointing to what issues are going on to me either. Your veterinarian and you have really done so much , even an X-ray!