You're sorta right but mostly wrong but you're wrong about
the wrong thing so you're sorta right.
You're right that serum PSA has a half-life but your 5 days figure is off by a factor of two. The usual figure quoted is "two to three days" but the values I've seen tend to cluster around 2.5. ( For instance, see table 2 here: Clearance rates of total prostate specific antigen (PSA) after radical prostatectomy in African-Americans and Caucasians
) If we use a 2.5 day half-life then after six weeks we'd expect something like a 2-16
reduction giving an expected PSA of 0.00274.
Another reason that surgeons like to wait a bit before checking PSA values after surgery is that it is not uncommon for small amounts of prostate tissue to be left behind after surgery but detached from its blood supply. Those little bits of tissue usually die but they can linger long enough to wreak havoc with PSA half-life analyses. So you're right that six weeks is early-ish but you are wrong about
Thing is, with Gleason 9 disease you generally don't want to spend a lot of time sitting around hoping it will go away.