I presume his cancer is sensitive to the PSMA scan, i.e. it did show up the cancer in the prostate? (Some prostate cancers don't register on PSMA scans at all.)
Your husband is young, and that changes things in my view, because you are looking for a long time in remission. I think a prostatectomy is unlikely to be curative, but it may push the can further down the road before another treatment is required, and even if it doesn't do that, it may make another treatment more effective. So while I wouldn't normally recommend prostatectomy for a G9 T3b, I think it might be worth discussing due to his age. It's a bit unusual, but I might even ask about prostatectomy + 2-3 years of hormone therapy, as if there are any remaining micro mets too small to show on scans, the combination of loss of the mothership tumour (prostate) combined with hormone therapy may have a good chance of killing them.
In the case of hormone therapy, you probably wouldn't know until about a year after finishing the hormone therapy when his Testosterone has recovered and his PSA will be indicative of any remaining cancer, if the treatment worked or not. If it didn't and his PSA is rising, then another PSMA PET scan would be useful to try and find where to target radiotherapy. There might even be some newer treatment options by then, but prostate bed radiotherapy plus aiming at any mets found would be an option (providing only a small number of mets found - this doesn't work for more than 3 mets).
The impact of this treatment on erectile function may be significant (nerve sparing might not be available with G9 T3b, but it depends on his specific case), and many men will seriously struggle with that long term, even if it doesn't seem important at the moment of a cancer diagnosis. Skipping the prostatectomy and going straight for radiotherapy might have a higher chance of preserving erectile function, but there are no guarantees either way. Being younger will help. I skipped a prostatectomy because it would not have been nerve sparing and probably not curative when I was diagnosed 5 years older than your husband, and I went straight for radiotherapy which was the right choice for me.
If he does go for radiotherapy first, I would ask about having HDR Boost (a combination of external beam to cover prostate, seminal vesicles, and pelvic lymph nodes), and a high dose rate brachytherapy boost to the prostate and seminal vesicles, where the known disease is, plus hormone therapy of course. This gets a high dose into the cancer, and a lower dose spread outside where it's likely to go next, and should mop up any micro-mets which are already there. Only specialist centres can do this treatment, so that may mean being referred somewhere else, at least for an opinion on if it's a good match for his diagnosis. Don't feel constrained to just the treatments available at your local hospital, but bare in mind the clinicians at your local hospital are unlikely be able to give accurate advice on treatments they don't do, so you would need to push for a referral for any treatment options they don't do which you are interested in.