It's the BOOTX64.EFI that the UEFI on that computer doesn't like, just grab the grubx64.efi from whatever the other folder is called and replace BOOTX64.EFI with it.
it should be, yeah. those kernel arguments that are on the steam deck image are soley for the OLED deck, and HTPC users use the same image as the deck. secure boot on desktop images should be fine