Just connect them directly to each other with a single Cat6+ cable, and statically configure them on a different subnet.
Such as:
- TrueNAS 2.5GbE NIC: 192.168.200.1
- Computer 2.5GbE NIC: 192.168.200.2
Then (on your client PC) configure your SMB shares, NFS shares, Rsync, SSH, etc, to use 192.168.200.1
You can also use this IP address to login to the web GUI.
You can even manually specify an MTU of 9000 for a small boost. (Only to be set on the respective 2.5GbE adapters on both ends. Don't touch the other adapters.)
(insert obligatory caveat emptor about RealTek cards here)