I was unsatisfied with the result, had to shoot some more to test and give it another shot. So in the afternoon I ventured out with my trusty Pentax 6x7 loaded with a roll of HP5. Few hours later I had shot through the roll, and it was ready to go into the lab box. This time I agitated the film in the soup for 9 minutes (instead of 6 in the previous attempt), followed by vigorous rinsing under tap water for a much longer duration. The result was very satisfactory: although there are purple stains on the film edge (that I believe was due to the film edge in contact with the spool), the images came out clean.
There are “some” stains in the following images but not as bad as what I got in the first round. Once I run out of the monobath I’m going to invest in individual developer and blix/fixer to have more control on the process.