This is a complicated scenario to unpack.

Efficacy of early salvage radiotherapy post RP (PSA ~ 0.2) is higher than if the patient waits until higher PSA levels after BCR. Adjuvant RT (when RT is considered a pre-planned part of the primary treatment) is even more likely to kill off all cancer cells but is also more likely to cause injuries to the healing anastomosis where the bladder neck and external sphincter were joined during surgery.

Even if a patient has an aggressive BCR with a PSA doubling time < 9 months, then unless he has persistent PSA, meaning his PSA never becomes undetectable post RP, then there should be time to allow the anastomosis to properly heal before zapping it with early salvage RT without significantly increasing the chance of secondary BCR at 5 or 10 years post salvage RT.

Another factor that should be considered in a post RP BCR is that the ability of a radiologist to read a PSMA PET/CT scan and correctly call a positive lymph node (LN) or cancer in the prostate bed increases as PSA values increase. The more cancer present, the more likely for it show up on the scan.
So if you do adjuvant RT assuming (but not knowing) there will be a BCR, then you can't target positive nodes and give them a booster dose of radiation. OTOH if there is no or very low PSA at that point then it doesn't take as much radiation to kill any extra-prostatic cancer cells in the treatment field.

Likewise, reading a PSMA scan post RP at a PSA level of 0.2 may result in some growths being detected correctly but may also miss some. This is usually accounted for in the treatment plan by applying lower doses to broader fields to account for unseen extra-prostatic cancer cells where you aren't certain cancer is present and more concentrated doses where you can see cancer on the PSMA scan.

So the bottom line IMO with post RP BCR -- there are a lot of tradeoffs to consider about when to re-treat, but you do have a very precise biomarker to know immediately if there is extra-prostatic cancer and as you track it month over month, you glean insight on the aggressiveness of the localized BCR or metastasis, whichever is the case. That's not feasible when you have BCR after primary RT with a prostate gland still intact...at least not that quickly and precisely.

With high grade PCa, the combination of brachy and EBRT as primary treatment has been shown to provide a higher likelihood of avoiding BCR at 5 and 10 years compared to RP alone. That is why men with high risk PCa believed (but not known) to be confined to the prostate often think of RP and salvage RT as part of the same therapy.

I realize all of the above is a lot to digest but hope it might help you navigate your decision process just a little.