I've had rabbits with bladder problems and never got to the root cause of it. Antibiotics helped a lot with the last one, but she couldn't be on them forever (I did try but vet said no more). I changed her diet (no pellets, lots of forage & hay), lower amounts of calcium-rich veg to reduce sludge, gave her more space and encouraged her to move about more to empty her bladder more frequently, extra water bowls with apple juice to flavour it to increase fluid intake, etc. It didn't really make a difference in her case. She didn't have arthritis, groomed herself, had a clean bum, partner rabbits were fine. She was prone to EC, though - and I suspect it was related.
I went through all the stuff mentioned on FHB's website - which is basically back to wild rabbit living. Wild rabbits don't eat processed or dry stuff and are always on the move and flicking urine around, so empty their bladder more frequently - which means there isn't the chance for sludge & bugs to build up.