Major issue has surfaced: I have updated the BIOS to v 1406, because I thought maybe that's getting in the way of the Thunderbolt card working in Mac (it is fully functional under Windows, but what's not??).
So after restoring the BIOS settings as per guide now I cannot boot anymore into Mac (still can into Windows). I run into an error probably related to the NVidia driver, despite I have stopped it from running by using the nv_disable=1 boot argument: I get the dreaded "busy timeout[0], (60s): 'AppleACPICPU'" message and the system stalls.
What should I do now?
I was thinking trying to run again from USB, but since today I cannot download the latest 10.13.6 installer, because Apple has just moved to Mojave and I have used the USB for file transfer and whatnot since I had a stable system for more than a week by now.
[EDIT] Second crisis is overcome by looking for a clean install of High Sierra. After following the breadcrumbs, I have landed on the right page of the full installer on this page, at nr 4:
https://support.apple.com/en-us/HT208969