Well, it was a bit of a hastle but I got it working on the freenas. At first it would not detect. It is realtek based, so I had to do some research. Mostly ppl in the know were just saying "Don't use Realtek," but I did find a compiled realtek driver for FreeBSD which I was able to install on the freenas and get the NIC to work. Speeds so far are good; from my Ryzen Desktop to the freenas, both with 2.5Gbps ports, I was getting the full expected speed.
Chewed this over, cost benefit wasn't there for me. My two "main" NAS units have both a unused PCIe slot, and two 1GBE ports that support teaming or bonding. $40 for a semismart 16 port switch that supports channel bonding, plus $30 for a two port Intel 1GBe (only intel based chips work with the intel driver, which is the only option afaik). Not 2x speed for single files, but does support load sharing where a second file would go between the other nics.