After some (sorry for bothering you) questions here, I was able to set up a mailman3 system which I regard as perfect for my use. Perhaps, you had similar problems with your installation, especially when coming from mailman2 and, like me, from majordomo......

I collected a lot of information which was presented in this mailling list archive here and also answers of my questions (thanks Mark and others)

News: https://github.com/Wikinaut/mailman3-tools has all of these tools.

README says:

Scripts for maintenance or setting-up of modern mailman3 instances

set preferences for all users
list held messages
delete held messages older n days

My setting is strictly as described in the documentation:

mailman3 + postorius/hyperkitty are installed via pip in an (venv) /opt/mailman/
My user scripts and a huge mailing list generator (not published here) are in a directory apart from that: user maillistgen home directory /home/maillistgen
user maillistgen has some sudo rights set uo in /etc/sudoers
a wrapper script is /home/maillistgen/run_mailman.sh which sets the correct path for Python, mailman3 and the (venv).

I hope, you like my tiny part of contribution and attribution back to Mark.