Many creators of new groups fail to fill out the Default Sub Settings. ...
I think that would lead to more trouble and confusion, both because it would make the Settings page even longer and because it would be mixing to conceptually different types of controls. I don't think defaults for member subscription settings belong mixed in with group settings.
It would be better, I think, to find a way to draw the new group owner's attention to the Default Sub Settings tab.
Perhaps a new group could have a green banner at the top of (each tab of) the Settings page with Settings, Default Sub Settings, Cover Photo, and Member Notices listed as bullet points under a heading reminding the user to review each tab (but not Export Group Data, and I'm really unclear on the Member Sync tab that seems to come and go). As the Update (or whatever) button is used in each tab that bullet item would be removed, until all four are removed and the banner itself goes away. In the banner, each bullet point could have a short phrase or sentence to indicate what that tab is all about.
... or if not all of it, at least those portions that relate to time and date.
I like this better. Those settings are arguably misplaced among the new member subscription defaults, as they initialize new user accounts instead. This does cause some confusion. Some new group owners do not have a firm understanding of the distinction between a member's subscription to (membership in) their group versus that member's account at Groups.io.
The settings don't really belong among the group settings either, but at least there are fewer of them so they wouldn't overload that tab so badly.