Well, the fact that your medical team has not tested your testosterone level is interesting. Generally you would want a baseline measurement, then around three months of starting systemic treatment, measure to determine if the systemic treatment with ADT had dropped your testosterone to castrate levels. That level used to be < 50, more and more it is < 20. If you come off treatment, then it's useful to measure testosterone for correlation with any return of PSA.

Staging is important in treatment decisions, are you say N1 or M1...

You don't provide a lot of clinical data so I'm going off "assumptions" based on your post. It seems you were diagnosed "de novo, had some type of radiation to the prostate combined with systemic therapy, ADT for three years, then came off treatment and a year later your PSA is "slowly" rising.

If my read is correct then and you say "no apparent metastases," then it would seem you had some type of imaging, if conventional CT and MRI, unsurprising it shows nothing. If it was a PSMA, same, at that PSA level, around a one-third chance of locating recurrence, less depending on your PSA kinetics.

If you stopped systemic treatment after three years, most urologists and oncologists would check your PSA every three months and likely your testosterone too. The PSA tests would give more clarity to he "slowly" rising in the form of PSA doubling and velocity - https://www.mskcc.org/nomograms/prostate/psa_doubling_time

If that PSADT is less than six months, a decision about going back on treatment may be necessary, greater than that, say 6-12, then that's the gray zone, greater than 12 months, possibly continue to monitor.

What's not in your favor is that GS 9, that is high risk and puts you in Grade Group 5.

It's time to have your medical team collect pertinent clinical data to inform both you and them in making decisions.

You have some:
GS
Grade Group.

Other you may want:
PSADT
PSAV
Staging - informed by PSMA imaging - N1, M1,...
Genomic Testing

With that type of clinical data, you, in conjunction with your medical team can make any treatment decisions - if to treat, with what, when, for how long and whether continuous or for defined periods, If the latter, have decision criteria for coming off and actively monitoring with decision criteria for starting back on treatment.

Kevin