(That being said, always putting "via Groups.io" in the From username sounds like a good idea.)

"via Groups.io" is added when the headers have to be rewritten due to DMARC requirements so it signifies that something specific has happened not just as an eye-catcher.

Referring back to your first message:

we can avoid spam from people who wrongly "reply to all" (as many are wont to do by habit),

If your members are doing something 'wrong' out of habit, isn't a bit of education required?

Andy