For anyone finding this issue on Google in 10 years or something:
I didn't completely fix the issue yet but it's definitely related to my digital AV adapter. Some settings related to the sync rate can cause this issue from time to time and switching it on and off in the settings of the adapter can alleviate the issue.
Did anyone try this on real hardware yet? Is it made to perform well on the GameCube or does it really only work well in emulators?
Edit: I just tried it out and it works great on a real GameCube.