I read up on it and it seems from 6.1 onward you can use ReBar, but you don’t set it in the UEFI. There is a Reddit thread explaining the new feature. The article mentions support from UEFI is preferred, but given your setup outright refuses to work with it, I would let it disabled. For example I ran the checks from the article on my computer and my host already uses the biggest available bar sizes. So no need to touch anything for me. Everyone would need to check their own setup though.
P.S. I just checked and my graphics cards driver reports ReBar working, without me having done anything. ReBar is not activated in the UEFI and I have not touched the sysfs configuration. I figure you need to leave it disabled in the UEFI if the setting hinders the boot process, but you might check if it is already working and if not you can try to set the option manually as explained in the article via sysfs.