Sorry, but I'm only on a standard 1080p monitor, so unable to test 4k.
The EFI folder was exactly as per Jake Lo's upload earlier in the thread, so no point reuploading it. Also Feizhai has kindly quoted it a few posts up.
And yes, I believe it was a APFS format disk, initially formatted this way using Disk Util after booting off the USB drive.
I did find I didn't have the latest A22 bios installed. Upgrading to that fixed the last "quirk" I was having with sleep not working right. Now it just works. It doesn't wake as fast as my imac, but it does do it.