Nice to see some folks still using the best Unix around, worked for this team also for 10+ years in the past ;-)
Sorry, but I am currently not on track whats the state of the sound system.

ALSA is in the ports tree, there you might have IMHO the highest chance to get something working.
1st I would ask in the ALSA community regarding the general state of HDSPe (AIO ?) support.

Otherwise you might have good or better chanced with a RME recording interface supporting Class Compliant mode.
But then you would require something like the UCX etc.

I remember that the Linux emulation in FreeBSD worked excellent and stuff was working these days even better than on Linux itself, as the code quality and multi processor support was in FreeBSD always superior. Maybe you have therefore luck with the ALSA application port.

Best out of the Box support you will find on a Linux system with this Linux distribution.
https://ubuntustudio.org/

Another good strategy might be to perform a parallel installation on Ubuntu Studio to see if and how it works and what the results are on a Linux System optimized for Studio work. Once you are confident how its works you can try this all on FreeBSD.

To make best use of RME recording interfaces I would use Windows. Windows is more efficient than Apple and with Apple not all Digicheck tools are supported. Global Record is very practical but only available on Windows.

The pragmatic approach would then be to use FreeBSD for daily work and Windows on a parallel installation for efficient recording out of the box without having to invest too much time and maybe running into frustration if it doesnt work or not work satisfactory.