Nick,if your PSA was 0.45 three months ago why were you not given treatment then. When my PSA was around 0.25 three years after surgery I was offered salvage RT, I asked for a PSMA scan but was refused, scans were a relatively new idea after recurrence in 2017.
My PSA did drop after SRT but then started to rise again, in the last 18 months they have found two separate cancerous pelvic lymph nodes, each of which were treated one last year, one this year. At 0.9 you are well over the lower limits for having a PSMA scan.
There is a risk that if something is found out side the prostate bed that choice of treatments can change.
Are you sure the figure is 0.9, do you have a list of your previous PSA tests.