Changing these settings in BIOS can create a mismatch with the O/S image and make it non-bootable. You need to install the O/S with these already set to what you want.
Fast Boot is fraught with caveats. I do not recommend its usage (and it really isn't necessary).
In the end, I reinstalled the OS from an SD card, that was detected as an UEFI boot drive, and deleted the bootmgr file from the Windows install folder, to make sure it was installed as UEFI and not legacy. It works fine now, but there's definitely something wrong with the way legacy boot drives are detected from the bios :/