I do not think, I can really help, but yesterday the auto installation setup failed for me, maybe it is a similiar issue for you:
I tried to setup a test setup and used the root on zfs auto installer, after setting up the partitions (in my case I chose UEFI+Bios option) 1. Efi boot partition, 2. freebsd-boot, 3. freebsd-zfs, the installer just exited and the computer restarted, it never got past that step to start the installation.
Are you sure, that the installer finished successfully? If it did, your UEFI should autodetect the bootloader in <efi-partition>: EFI/BOOT/BOOTX64.efi
If it is only about setting an entry to active, you can use the efibootmgr tool (
https://www.freebsd.org/cgi/man.cgi?query=efibootmgr&sektion=8&manpath=freebsd-release-ports)