Allow internal sharing but require approval for external sharing

This is an interesting question. Not that I have the answer, because I don't, sorry.

As far as I can understand, you are not talking about the usual access request process, where an outside user requests access to a given site.

What you are talking about, is a user, who already has access, wanting to share something with someone on the outside. And in order for that to happen, someone has to see and approve that sharing request.

That is definitely interesting - it could make sense in certain situations. For us, in many cases, I would be worried that it would cause to much work for site owners and they would become a bottleneck. I mean, how do you handle the situation where, someone using the Outlook desktop client, wants to share a link to a file? Does that person have to request permission to send the link? And when does the site owner have to respond? What happens if the site owner can't respond immediately? Then you can't send the link?