I don't have an answer, but with no one else replying I thought I would reply to express sympathy. It really sounds as if you are having a tough time.
I suspect your situation is so unusual that very few have similar experience. Certainly my own radiotherapy had relatively mild effects on my bladder; it was salvage RT after surgery 6 years earlier and I already had mild incontinence problems (one pad a day) which got a bit worse but still manageable. It certainly didn't affect cycling, I cycled to my radiotherapy sessions for the first three weeks of my 4-week treatment, and while I was then affected by the tiredness and lack of energy most people report I returned to doing most things after a few weeks.
You will find quite a few threads on here about returning to cycling following surgery, and while it is a different situation there might be some clues. Needing a catheter after RT is unusual - yours are the first time I have read about it - but it is universal after surgery when a catheter is needed to allow urine through the join where the two cut ends of the urethra have been reconnected and are still healing. It is worth checking those threads, but I get the impression most surgeons suggest allowing around 3 months following removal of the catheter and some are more cautious than that.
Good luck. It will clearly help your quality of life to be cycling again, but that description of urine retention suggests a repeat mustn't be risked. The urologists are the experts, and hopefully they can give more optimistic guidance as things move forwards.