I think I solved the issue after all. When I said that I had the iGPU disabled from UEFI, it turned out to be untrue. Which I found out when I restored my Win10 setup only to find out there were both video card adapters active, Nvidia and AMD. Recheck in UEFI showed the iGPU (AMD) was indeed in disabled state , (a bug in UEFI, I suppose?) but setting the video adapter settings once more and rebooted to Windows, control panel showed only Nvidia adapter present, meaning the AMD iGPU now really disabled.
Then reinstalled LibreElec 8.00.0, which is now working beautifully.
So the root cause was having two video adapters present in the system, I guess.
Live and learn.