In case anybody happens across my post above and faces the same issue:
The FreeBSD install instructions on the FusionPBX install page work just fine. However, please note that Fusionpbx cannot be installed inside a FreeBSD jail that way. It must be installed on either a bare metal machine or inside a bhyve vm. I assume that with additional work it can be installed inside a jail, but not from the install script.
I succeeded in setting the system up on a fresh FreeBSD13.0 bhyve guest. The host is running FreeBSD12.2.
Thanks to everyone at FusionPBX for the hard work on this project.