I have started testing your thoughtful suggestions. Some findings:
I think I have the #noemail hashtag working properly using the https://999999.groups.io/g/main/edittag/ page
But as pointed out in https://groups.io/static/help
Possible solution: Special address in reply to moderators group or special sender email address where members can see all the reply messages. I looked into setting up a firstname.lastname@example.org type subgroup for replies but could not get that going.
Anyway, I'm learning about groups.io slowly but surely and will keep plugging away.