not able to boot OpenCore / not shown as UEFI partition

So I've been working on updating my 10.14 setup to BigSur for a few days now, and I'm stuck.

I have 2 SSDs (840 and 860), on the 840 there is the original working setup (Clover + 10.14) , and I used CCC to clone it to the 860 (also copied the EFI partition).

I first tried a fresh install using OpenCore on a USB, which worked until halfway through the installer. Then I tried a lot of different things to fix it, none worked, until at some point none of the drives could boot. I reset the NVRAM, then booting from the 840 worked again, but not from a USB drive.

So I started a different approach, did a fresh clone of the 840-->860 (including EFI), but I cannot boot Clover from the 860. It works to boot to Clover on the 840 and then select the OSX on the 860, but somehow the disk is not shown with a UEFI prefix in the boot drive selection (and non-UEFI disks are not working, see screenshot). When I select one of the non-UEFI ones it says "reboot and select proper boot drive". Also I don't know why the same disk shows up several times, it seems like they replicate (it becomes more and more entries the more often I boot).

I'm really stuck and would love to have some hints on where to look. Preferably I'd just do a fresh OpenCore install, but since it's not booting anything but the 840 I'd be happy for any kind of workaround/solution.