This requires you to run two operating systems simultaneously on your PC. In order to do this, you need two individual graphics cards in your PC (any cheap GPU will do for your work screen), then you need to purchase a special virtualization software ($60), configure it correctly and reinstall your operating system twice (ideally on two separate hard drives or SSDs), which means you need another Windows license if you want both virtual machines to use Windows.
Linus Tech Tips did this a while ago, starting with two users per PC and then gradually increasing the number in follow up videos. Here's the first video with a detailed and easy to understand how-to guide:
https://www.youtube.com/watch?v=LuJYMCbIbPk
Note that this was three years ago, so the software and its settings may have changed since.
Your PC is powerful enough, but I would suggest at least doubling the amount of RAM, since just 4GB per operating system would be too little, especially on the gaming side.
This is a major undertaking that takes hours to properly set up and test, not to mention the time required to backup your data beforehand and install your software afterwards. There is no simple software you can just install and then two people can use the same PC.
So one thing that's really interesting to me is that I didn't know how big unraid was. Every time I'd see it mentioned in various places
A bit more familiar to me is MAAS and OpenShift -- have you given any of those a try? I know I avoid OpenShift like the plague because it just has too many moving parts but it feels like MAAS might be able to give Proxmox a run for it's money someday
When you are able to buy a new computer in the future, look into Unraid's Mumti-seat options because you will get more performance per dollar buying one powerful PC to run two virtualised seats from than if you bought two mid-range rigs
Unraid https://unraid.net/product is an alternative to freenas that I've setup as a time machine server. Here's a (long) video showing how <https://www.youtube.com/watch?v=5J955nNIdo0>. You can trial unraid for 30 days.
I'm not sure about being able to manage multiple time machine sources so that they can talk to each other. For troubleshooting time machine, I've used backuploupe https://www.soma-zone.com/BackupLoupe/ to dig deep into my backups.
If you only need enough space to get away with the internal drive(s) and maybe one external USB3.0 drive then sure - they're a great option. I use an i3 NUC like that for running docker containers like nginx, unifi, deluge, sabnzbd etc with no issues.
If you're after more of a full blown NAS setup with a decent amount of storage, i'd probably recommend getting something bigger if you have the space if only for expansion capability. Otherwise you could end up like this guy. You could definitely go the USB attached DAS/enclosure route - which will most likely be absolutely fine, though would maybe limit options if you were wanting to move to something like unRAID.