Metastases may have occurred
before you ever had your RP, or after you had it and before you started Lupron. When your PSA jumped up from 0.365 to 2.1, it may be that metastases were responsible - hopefully, those metastatic cells were trapped in your pelvic lymph nodes. Hopefully, any systemic cancer cells were hormone sensitive and were killed off by the Lupron and your immune system. It is unlikely they would have occurred after you started Lupron, unless it was a population of castration-resistant cells too small to increase your PSA. One scenario is that the cancer is still in the prostate bed and possibly the pelvic lymph nodes as well. That would mean you still have a chance at a cure. It's not a great chance, but it is a chance.
To gauge your odds that salvage RT might still be curative for you, you can use the following nomogram:
riskcalc.org/ProstateCancerAfterRadicalProstatectomyNew/Their nomogram doesn't include PSADT, which is surely a negative factor in your case. It's also unclear what "pre-RT PSA' you should use - probably 2 (it doesn't accept higher values). In your favor would be whole-pelvic treatment and a higher prostate bed dose (~70 Gy).