If anyone using #nextcloud needs a #maillist app then I got what you need!
I also was using nextcloud and needed a mail-list app, but there didn't seem to be one so I made it:
https://github.com/revpriest/listman
Doubt it'd scale to thousands of list-members, but I should be so lucky to have a list of thousands of members. I'll worry about that if I ever get to a few hundred.
It should cope well with a few hundred I think.