This is the default behaviour on Apple monitors.
I'd also like some way to control brightness of the internal display when using MonitorControl with an external monitor.
Right now I am using this one instead of MonitorControl to achieve that:
https://github.com/Bensge/NativeDisplayBrightness
It has no UI, it's not supported anymore, but it simply controls brightness of all my external monitors at once using F1/F2. Which is actually everything I needed.
So Fn keys remain free to control internal display, and F1/F2 (for me actually Fn+F1/F2) control the external ones.. and it just works 馃憤
This is something that I plan on adding together with #118 but it requires a lot of rewriting under the hood.. and I'm very limited in time. I made some of the first steps in previous commit linked here (46d7cb6) rewriting the display managing logic.
The project was long overdue for some refactoring of the display logic anyways..
Most of the code refactor is done, the internal display respects the isEnabled option inside settings and should also work with ctrl modifier (in that case no other displays will be controlled).
Just waiting for https://github.com/the0neyouseek/MediaKeyTap/pull/5 to be reviewed/merged right now. After that I'll do a few more final things and open a PR for review.

@JoniVR Merged :wink:
Thanks for all your hard work on this
Pull Request has been submitted!
For the people that know how to, please feel free to test/review it and report any issues since a lot of the display logic has changed, there's always a bigger chance of introducing new issues. It's pretty hard to test with only one configuration so the more people that test it the better.
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@JoniVR Merged :wink:
Thanks for all your hard work on this