This is another area of active investigation with a lot of good answers but no "right" answer (IMO).
My general approach in these situations is PSMA-PET (Or Axumin-PET, more on that in a second) and if negative and the patient is relatively fit and motivated, 3 months of ADT --> prostate bed XRT --> 3 more months of ADT, then monitor. I've seen people go forever on ADT or up to 2 years, but in general, if they get to undetectable at 6 months, I'll give them a break and do intermittent ADT.
If they're asymptomatic though, or not particularly fit, or don't want ADT or , then observation isn't unreasonable. You can also do XRT alone or ADT alone, recognizing that both are inferior to combined therapy but that may not be relevant in an 87yo vasculopath with cirrhosis.
What to do after this approach when you don't get a complete PSA response is also variable. Some people give an ASRI, some people use abi/pred, some just ride it out and see what happens.
I think the key in managing these cases is to find out what's most important to the patient in terms of QOL, PSA, treatment intensity, etc and tailor your treatment that way.
The PET tracer issue is getting complicated in prostate cancer (and other cancer types as well). There is data in the nuc med literature that PSMA is more sensitive for distant mets and Axumin more sensitive for prostate bed recurrence. I honestly don't know what to do with this in the real world, other than spend too much money on scans.