There are some rare side effects of TURP but it is usually a low side effect procedure. The main side effect is retrograde ejaculation, where you ejaculate backwards.
I do not know, but would guess this would not interfere with RT. You would have to heal from the TURP first. It’s up to the doctor. I have never heard of anybody getting a TURP as a pretreatment prior to radiation.
RT can cause the prostate to inflame and if you already have voiding symptoms it could make it worse. Some men have to self-catheterize after RT for a bit. My guess is this is what the doctor is trying to avoid.
If you are concerned you might consider a second opinion. Whether to have a TURP and when you can start RT, is definitely in the realm of “doctors preferences”, where you might get different opinions from different doctors. There is no one right answer.
I myself would not want to wait to get a TURP and then heal, before the RT, with a Gleason 9 which can be pretty aggressive, but that would be based on the doctor convincing me that the RT side effects could be adequately managed including any problems with urinating.
Best wishes
Gene