I wouldn't wait for your cancer to return to the point it shows up in a PSMA scan. You're an interesting case in point that even going over 0.1 doesn't necessarily mean you need salvage treatment. The most common criteria I've read is that two successive rising readings above 0.03 could be a decision point if you want to do salvage. Bouncing around without two increases in a row at least 3 months apart would not be a reliable result to go through with salvage. It also boils down to how conservative the Urologist is with salvage treatment. Some may recommend having it done once you have two successive results rising over 0.03, others like you elude to may recommend doing nothing until you have more than 2 rising results and get to 0.5 but by today's standards, that may be a bit late.

Noteworthy, very early SRT (PSA 0.01 to 0.2 ng/ml) was associated with a twofold decrease in biochemical failure, use of salvage ADT, and distant metastases compared to early SRT (PSA between 0.2 to 0.5 ng/ml) (87). Similarly, Fossati et al. also concluded that SRT should be given at the earliest sign of PSA rise, and even more so in case of adverse pathological findings

If you do get SRT, make sure it is the latest "MRI guided" radiotherapy.