@tobiaseigen some stuff here gets very complex.
The change of allowing site_contact_username to be a group has one very complex scope to it that makes this a no go.
At the end of the day a group can easily get all the notifications for a message (by being in topic allowed groups), but the message has still to originate from an actual user.
I think to solve the OP here we should simply add site_contact_group_name if that is specified the site contact emails should all also go to the group. It is a trivial change with no new UX.
I don’t want to allow site contact username to be a group cause then we need to send the messages from an account, system is not a good fit, random user from group is not a good fit. So you should be allowed to choose here.