Could use it kind of like an extra monitor with something like Barrier.
Could use it like a home assistant for a kitchen or something, but I don’t know if there’s any good privacy respecting software for that ATM (looks like MyCroft went bankrupt).
I used an old laptop I had laying around for controlling a Maslow CNC. Could also use a laptop to run OctoPrint or something.
I personally use a dual core pentium with 16GB of RAM. When I first installed TrueNas (FreeNas back then), I only had 8GB of RAM, but that proved to be not enough to run all the services I wanted, so I would suggest 12-16GB. Depending on the services you want to run any multi-core x86 CPU that allows 16GB of RAM to be used should be adequate. I believe TrueNas recommends ECC RAM, but I don’t think using consumer grade RAM and hardware has caused me any problems. I’m also using an old SSD for the system drive, which I is recommended now (I used to use 2 mirrored USB thumb drives, buy that’s not recommended anymore). Very importantly, make sure the HDD(s) you get are not shingled drives; made that mistake initially, and performance was ridiculously bad.