The crucial part to booting FreeBSD on my 2018 Intel Mac Mini (macmini8,1) was in the FreeBSD wiki:
$ mount -t msdos /dev/disk0s1 /Volumes
$ bless --mount /Volumes --setBoot --file /Volumes/efi/boot/BOOTX64.efi --shortform
$ diskutil umount /Volumes
$ reboot