Successfully installed 19.7 on a 2008 Mac Pro; subsequently applied the 19.7.9 update as well. Did the initial install via ssh because the system console's video becomes corrupt after the OPNsense live image (and OPNsense installed on an internal drive) begins to boot, i.e. the display is readable while the boot manager is running but once OPNsense begins to boot the console is unreadable (images attached). I'm not familiar with configuring FreeBSD's console settings and wondering if/how the console configuration can be managed through the OPNsense web interface.
Maybe is has something to do with the EFI. The 2008 Mac Pro doesn't use UEFI.
Have you tried VGA? You can set that in the configuration on the webconfig.
I had a 2008 Mac Pro but never installed FreeBSD/HardenedBSD on it.
"System | Settings | Administration | Primary Console" is set to "VGA Console". I tried changing that to "EFI Console" and rebooting, but get the same result. Not sure if this is useful information, but if I log in via ssh, start the shell, and issue "vidcontrol -i mode < /dev/console" all I get is an empty table.
Hm, in that case try a different videocard? You might miss the EFI boot screen, but at least you have OPNsense screen.