I was thinking of using FreeBSD for a large filer as well, but then I learned that you can run ZFS on Ubuntu, which should have better driver support than FreeBSD (at least I would think so). So you may want to consider Ubuntu with ZFS instead.
https://www.howtogeek.com/272220/how-to-install-and-use-zfs-...
"Better" is a little subjective. Linux has more drivers, true. But you only need one driver, for the HBA you use, and is the quality of that one driver better? I bought an LSI HBA specifically because the FreeBSD driver for it is known to be good (and it also works well with Linux).
Drivers aside, ZFS on Linux is rather more rough around the edges than on FreeBSD. Having used both, I'd place more trust in ZFS on FreeBSD. While I've not had dataloss on either platform, I have seen odd glitches on Linux which required rebooting to resolve, like zpool getting stuck in D state after some pool operation, and it failing to mount filesystems after rebooting until I manually reset the mountpoints for little reason I could see, and excessive memory usage which has frozen the machine on a few occasions when under heavy load. It's also rather more featureful on FreeBSD; the Linux implementation has a number of annoying restrictions and missing functionality which aren't essential for the basics, but make it much more pleasant to use. Like not requiring root, priv delegation, NFSv4 ACLs, NFS export, ARC integration into the kernel memory management.
Drivers aside, ZFS on Linux is rather more rough around the edges than on FreeBSD. Having used both, I'd place more trust in ZFS on FreeBSD. While I've not had dataloss on either platform, I have seen odd glitches on Linux which required rebooting to resolve, like zpool getting stuck in D state after some pool operation, and it failing to mount filesystems after rebooting until I manually reset the mountpoints for little reason I could see, and excessive memory usage which has frozen the machine on a few occasions when under heavy load. It's also rather more featureful on FreeBSD; the Linux implementation has a number of annoying restrictions and missing functionality which aren't essential for the basics, but make it much more pleasant to use. Like not requiring root, priv delegation, NFSv4 ACLs, NFS export, ARC integration into the kernel memory management.