So I did do a bit more research, and apparently Realtek and Aquantia controllers are supported in Truenas Core with the system tuneables. On my Freenas in current version, I did get the Realtek NIC rosewill card working with the driver and updating the loader.conf file. Obviously using these would not be hassle free, but I don't mind tinkering and learning, which can be part of the fun.
That said, I would love to get a good Intel 10GbE-T based NIC for a similar price if possible. The concern though might be warranty. Due to the systems they would go in, they should probably be PCIe gen 3 x4 interface. Though the NAS is using a board limited to PCIe gen 2, this shouldn't be an issue. Also, I would prefer if the NICs also support 5GbE-T and 2.5GbE-T.
For reference, the NAS is using a Gigabyte GA-X58-UD5 motherboard with 18GB of RAM and a Xeon X5660. The data drives are 5 WD Red Pro 4TB drives in Z1. They are connected to the SATA II based ICH10R south bridge on the board.
I would prefer to use existing copper CAT6 cables, and use the 10Gb ports on my existing switch, with compatible transceivers, which I could use recommendations on. Of course, this is assuming I actually upgrade to 10Gb. Is it possible that 5Gb network connections would not bottleneck transfers to/from the NAS, given the drives and system specs? This might be a way to save money, though if going for any upgrade, it seems best to right to 10Gb.
All that said, any links for a good deal on said equipment? Thanks again for advice.