Or, one might consider a switch to a different list manager entirely.
Sympa is really excellent for handling large numbers of lists, handles
DMARC issues, doesn't send cleartext password reminders, is incredibly
well supported.
I think Sympa is just dandy, and am in the process of switching to it
from mj2, but its DMARC hackery isn't much different from Mailman's.
It does have remarkably good DKIM and S/MIME handling.
I have a back end shim that rewrites mail addresses as required to
avoid DMARC while remaining deliverable, e.g., if a message is from
marissa(_at_)yahoo(_dot_)com, it rewrites it to
marissa(_at_)yahoo(_dot_)com(_dot_)dmarc(_dot_)fail and
sets up a temporary local forward.
It's grody but it works remarkably well. Anyone wants it, just ask.
It's written in perl and the full forwarding groditude uses a litttle
daemon (also written in perl) that will need some rewriting if your
MTA isn't qmail.
R's,
John