Hello Magnus
Yes, I once built one with an Atmega8, which also works with mirror release and has a timer.
But I can only use it locally in front of the telescope.
I actually wanted to save myself the additional effort and control the camera and telescope via Kstars / Ekos.
I read your thread a few years ago, it looks like you got stuck.
I know that it worked years ago with an old version of Gphoto. But that was certainly 10 years ago.
If the translation is a bit bumpy, I apologize.
I use google translate.