I was not prepared for the multi-gigabyte armada of Python libraries and
their dependencies that is the Debian "mailman3-full" package. Merely
launching the contraption brought our VPS to its knees.

The deal is that "mailman3" is the core, and you need add-ons in order
for it to be useful. The standard add-ons are "Posterous" [sic?] for
web-based list administration, and "Hyperkitty" [sic] for archive
browsing and indexing. I didn't dare look what is being used for the
indexing backend. All the web stuff is written in Django, which is gnarly.

In brief: Is there anything else we could use?

The Debian project uses a Procmail add-on called SmartList to run its
many lists. Of this I heard, among other similar remarks, "Procmail is
what you get if Sendmail got beaten up in an APL bar."

I found a Go program called ListMonk, which was light and had a great
UI, but apparently it's only for sending newsletters. Subscribers cannot
use the lists send mail to each other. I did not know the term "mailing
list" is ambiguous nowadays.