OpenMediaVault VS trueNAS

You might want to consider the previously mentioned MergerFS and SnapRAID, or MDADM and LVM for your setup. OpenMediaVault [https://www.openmediavault.org/ is a solid choice in this regard. Additionally, if you're planning to run something like Proxmox, you could look into deploying Starwind CVM on top of it. Pretty much like in this guide: https://www.starwindsoftware.com/resource-library/starwind-virtual-san-vsan-configuration-guide-for-proxmox-vsan-deployed-as-a-controller-virtual-machine-cvm/ .

You would better to install OMV instead. It's a ready to use fileserver solution with web interface.

So I'm working on a similar setup with an ancient desktop my in-laws were getting rid of. I installed OpenMediaVault directly to the hard drive (it's so old that I assume it wouldn't work too well as a hypervisor), with a 4TB external HDD attached. OMV supports Docker by means of a plug-in and I'm running a Jellyfin container with no issues at all. I'm still manually downloading everything to the HDD but *arr containers are my next step, as well as setting up a gluetun container to route all that through a VPN.

That is possible (it's automated to a certain extent). It also depends on the NAS OS. Unraid, for example, can work with drives of different sizes while using the largest one for parity. TrueNAS uses ZFS so you won't be able to just add a drive to your RAIDZ and expand it. There's also openmediavault: https://www.openmediavault.org/ that uses Linux Software RAID and you can add a new drive there but you can't combine drives of different sizes. And there's also Starwinds NAS that supports both Linux Software RAID and ZFS: https://www.starwindsoftware.com/san-and-nas. Plus, I would avoid SMR drives in RAID: https://nascompares.com/answer/list-of-wd-cmr-and-smr-hard-drives-hdd/

I'd put it like that: if you have a spare PC, then it's definitely worth trying. I would also look into openmediavault (minimal RAM needs): https://www.openmediavault.org/ and Starwinds NAS (supports both ZFS and Linux software RAID): https://www.starwindsoftware.com/san-and-nas

Second, I want to turn the OptiPlex 9020 to a nice automated Jellyfin Server, with *arrs included. I was thinking using truenas scale for this since I found a good guide here: https://github.com/imjustleaving/trueNAS/wiki/A-Guide-to-go-from-a-bare-metal-TrueNAS-Scale-install-to-a-Fully-Automated-Media-Server

I just found a project called Unpackerr which sounds like an absolute dream for me, it would fix my unpacking issue. I have Truenas Core installed, and found a guide for installing on Truenas scale here. During the container configuration, you need to set up some environmental variables.. is this possible to do on Truenas Core?

because this question gets asked so much (and bc ive been through this myself) i recommend this guide which helped me tremendously.

also, since u are so new to truenas, u should def be watching some youtube stuff on it. the webgui is pretty smooth, but not everything is super intuitive. plenty of stuff out there on how to do basic config so u get a little more comfortable. also lots of guides like this that do easy step-by-step stuff.