
Okay! We ended up going with Gaggle Email (https://gaggle.email)
Gaggle has a really nice free tier and is up front about their financing (even at the free tier).
It allows for simple joining, unsubscribing, administration, and moderation via web app.
It allows for newsletter sendouts.
It allows for privacy for folks that have joined the newsgroup. With a setting toggle, no one in the group can see anyone else's email address.
Most importantly, for me, it allows group members to email one email address to email the entire group and it allows folks to reply to that email to carry on a thread via email.
Cheers to @zebbm for the lead!