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.
If it works, I don’t update unless I’m bored or something. I also spread things out on multiple machines, so there’s less chance of stuff happening like you describe with the charts feature going away. My NAS is pretty much just a NAS now.
You can probably backup your configs/data, upgrade, then deploy jellyfin again, restore, and reconfigure. You should probably backup your data on your ZFS pool. But, I recently updated to the latest TrueNas Scale from ~5 year old FreeBSD version of TrueNas and the pools still worked fine (none of the “apps” or jails worked, obviously). The upgrade process even ported my service configurations over. I didn’t care about much of the data in the pools, so only backed up the most important stuff.