@darth
did test that as well, but didn't help as the kernel didn't boot up at all.
In the end, after also trying to reach out to ASUS Support (and ofc without any proper solution from their side), I did replace my custom built, noname, (trash) motherboard and now it works flawless.
So, I think it was something with the ACPI/DSDT Tables f'd up on the old MoBo and ASUS just couldn't (or didn't want to) fix that...