I am someone who had PSA rising following prostatectomy, and had hormone therapy and salvage radiotherapy.
My oncologist said the same as LynEyre reports above. In her opinion PSMA was unlikely to be meaningful with PSA less than 0.5 - but the salvage RT was likely to be most effective if started sooner than that. I was happy to start RT without a scan.
Certainly there are side effects, as there were with surgery. The hormone therapy wipes out libido - so is a cause of ED - but is supposed to be reversible though it wears off very slowly and I am not there yet (that was with Zoladex/goserelin, other medicines may be better). The whole point of radiotherapy is to be pretty nasty to the cancer cells, and there is a certain spillover to adjacent body parts which meant I had a rather sensitive bowel and bladder for the first few weeks as well as feeling generally shattered.
Good luck.