As I noted in another reply, looking on the Debian tab rather than the pip tab was a large part of the problem!
My services make use of various external things which are present in my specific environment, so they are no use to anyone else, and packaging them as extensions didn't (and doesn't) seem to make sense.
I did not mean to imply that the venv PITA was in any way the weewx folks' fault - just expressing my frustration. The previous time Debian and Pi upgraded the OS (to Bullseye), it took me a couple of hours to configure the updated OS for and with all the (substantial amount of) home monitoring and control software that runs on my 'workhorse' system. This time, it has already taken me days of work, and I am still not done!