Solved, with some help from ChatGPT as well. It made me aware of the fact that you can leave “Gateway” and “DNS Servers” blank if not connecting to the internet.
So, just manually set IP in TrueNas Scale. You can do this either through the Web UI if you connect via a router, or through the console (add a “/24” at the end so it’s in CIDR format). I used “192.168.2.2/24”
On the PC network card, in TCP IPv4, use a manual IP address. I used these:
IP: 192.168.2.1
Subnet mask: 255.255.255.0
Gateway, DNS servers: leave blank.
Testing right now with only 1x 18TB Toshiba MG09ACATE, will move my 3x RAID0 ones inside the NAS tomorrow.
A big shoutout to @dan for giving me some help, thanks!