I think the combination of rising PSA and positive margins mean you probably will need some more treatment, I'm sorry to say.
Your doctor is right in that your level is still too low to have much chance of showing up on even a PSMA PET scan.
This means, you could go into blind prostate bed radiotherapy soon, or you could wait until the level is high enough to have a chance of showing on a scan (but still no guarantee), just in case that shows something else that needs including, such as pelvic lymph nodes. Given you had a positive margin, prostate bed is very likely the location, but doesn't rule out it also being somewhere else.
Your Gleason was 4+4 which is the high end of intermediate aggressiveness, so you might want to bear that risk in mind too, in that the chance of spread increases if you wait a long time. The decision is a gamble.
Sorry you find yourself in this position of having to make yet more decisions about the disease, and wishing you all the best.