WikiSuite VS OpenMediaVault

Before proxmox I was using kimchi: https://github.com/kimchi-project/kimchi

I have looked into Cockpit. I agree that it is probably the best way forward to manage virtual machines (Kimchi seems nice too).

Cockpit or proxmox comes to mind. Quick Google search also came up with Kimchi.

Kimchi is an open-source HTML5-based KVM management tool that is designed for ease of use. This web-based virtualization management platform provides an intuitive, flexible interface that displays and provides control of all the VMs running on a system. Allows you to manage most of the basic features you need to create and control a set of guest virtual machines.

Maybe look at kimchi? It has multi-user support but I don't know to what extent.

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

Your current PC looks very decent for a lab. Proxmox on it, then one Linux VM with Docker, another VM for NAS with something lite and easy like openmediavault: https://www.openmediavault.org/ or Starwinds NAS: https://www.starwindsoftware.com/san-and-nas and Plex separately. And other stuff in VMs or containers.