And the instructions in UEFI FreeBSD Wiki advise to copy loader.efi, not boot1.efi. This is where I took my own method (I gave this link in my second post). Nevertheless, olli@ was right, the official method speak of boot1.efi. I tried again with this last file and it works. Reading quickly the respective codes, I can't figure out why both files are actually working. Maybe loader.efi works only if the efi partition (ESP) is on the same disk than the system partition?