While it's true there are many other forms of group communication, most of them involve the use of a specific software which you need people to adop, install on their computer, phone etc.,...whereas pretty much everyone has an email address and know how to use it and so for some groups mailing list are still very much used and probably most accessible (and to be honest I still find them pretty good for group comms).

VERY GOOD POINT !

Does Listmonk not cut it ?
I’m not really into Mail List software so maybe they’re aiming at different functionality

Yes, they're much different and do not serve the same purpose, as in it is not a newsletter nor autoresponder nor email marketing emails stuff, it is a Mailing List server.
It can be a little hard to grasp as it is more seldom used nowadays.

You can kind of compare its use with a FORUMS software, only it works exclusively through email. You manage it through commands sent to the server by email. One subscribe and unsubscribe through sending a command to the email list you wish available on the Mailing List server. The discussion goes through email, you add your comment by replying to the list and everyone on the list receives it by email and so on...

This software is among the oldest one of its kind I believe, so it's pretty solid, reliable and powerful. On the other hand, it should be pretty much useful if one finds a use for it, but personally I wonder if it is still relevant as a communication software nowadays with all that exists for group communications.

Do I read correctly that it would need Mailman Core and Postorius for the web interface ?
2 packages in one container ?

I've no idea to be honest, but as I understand it this is the main issue at present.

Please upvote the OP if you agree!

Source code
https://gitlab.com/mailman/

GNU Mailman - Wikipedia
https://en.wikipedia.org/wiki/GNU_Mailman

Open Source Self-Hosted Mailman Alternatives
https://alternativeto.net/software/mailman/?license=opensource&platform=self-hosted