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You can take a look at Pinta[0] I don't recall if it's a clone or a port of Paint.Net, but works for me as a quick tool... the OS integration (open with) needs some work though. It uses Mono as a portable runtime and is decent enough, again for most work. IMHO easier to use than Gimp, though not as feature rich.

https://pinta-project.com/pintaproject/pinta/

You don't need it on a mac. You can take a screenshot of a part of the screen only, and it saves it to an image on your desktop, you can send to anyone.

Anything similar to Paint in general, not just this specific use case. Paint covers a lot of uses that Preview doesn't.

I'm liking Paintbrush: https://paintbrush.sourceforge.io/