After High-Dose Rate (HDR) brachytherapy, PSA levels typically decline gradually over 1–2 years, often reaching a nadir (lowest point) of < 0.5 ng/mL or lower within 18–30 months. A "PSA bounce"—a temporary, benign rise of >0.2 ng/mL—is common within the first 1–2 years, frequently mistaken for recurrence..

as long as your PSA is dropping, gradual is ok...you have a somewhat moderate risk PC anyway, but seemed to be present throughout prostate. SO that might be what is slowing PSA drop..but good news it is dropping...test every 3-4 months and move on with life if you are not having any other problems/symptoms !