If I were to partition a /dev/sdb1, place all the needed root/home files, then go to my own /boot/efi folder and put the files there with whatever UUID I need, would it then be bootable?
Yes, the "right place" being an EFI FAT partition for the shimx64.efi and grubx64.efi bootloaders, along with the grub.cfg (all in EFI/ubuntu, and copies in /EFI/Boot with shimx64.efi renamed to bootx64.efi. Your root should be on an ext4 partition. The only UUID change needed is in the EFI partition's grub.cfg file to use the root of your system (assuming you just copied the whole EFI from another system. It's really pretty simple, and will even boot a legacy install of the root this way (not secure boot, but UEFI). Surprisingly, even the legacy /boot/grub/grub.cfg will work for a UEFI boot. You can add the /boot/efi and fstab entry for mounting the EFI, but that would only be needed if you tried to update shim/grub, (assuming you have the efi versions of the packages). I've been running a disk set up this way (legacy install, UEFI files added from another system) for years without issue. I did add the /boot/efi and fstab entry, but never bothered with the shi9m/grub efi package change.