Restricting unauthenticated sharing / Anyone links to certain users or group

+1. This is such a common requirement and sad that there is not a straightforward way to break whatever inheritance(s) are in place and 'do it like NTFS permissions'. 

That said, because you have 'Allow only users in specific groups to share externally', this means that only the members defined in that group will be able to share 'externally' and, to your point, will honor that ONLY based on the slider bars shown in your top screenshot unless..... Not exactly what we are after, and ALWAYS so much the case with the O365 platform.

Then, take into account this little doodad, which makes things EVEN MORE CONFUSING in Admin Center, that more or less implies per user setting, but sadly, cannot be invoked as such.

I suppose you could define two groups, as indicated here:

https://learn.microsoft.com/en-us/sharepoint/manage-security-groups

setting each respectively as:

• Authenticated guests only (default)
• OR
• Anyone