Aside from Mark's advice for the stock Mailman, there's also this patch:
https://gitlab.com/mailman/mailman/-/merge_requests/1161which allows Postfix to query Mailman's Postgres database directly. This patch isn't ready for application to the mainline for "cosmetic" reasons (see discussion in the MR), but it is used in production at a very large site as is for 6 months now.
With a little separate program, there is also the LMTP callback method documented in the installation documentation. There it takes advantage of internal support in Exim4, but could be implemented in 20 lines or so of Python which takes a TCP connection from Postfix and passes it on to Mailman's LMTP server, and reports the result of the RCPT TO command then QUITs. Finally, you could also use the REST API, but that means exposing what is normally a weakly authenticated unencrypted API on the wire.
Steve