Check out Duplicati. It’s cross-platform, can back up to a local disk, network share, or any of a number of cloud services, and can restore individual files, folders, etc. You can also specify storage intervals (e.g. keep dailies for the last week, weeklies for the last month, monthlies for the last year, etc. and restore specific versions of your files.
Yup, TrueNAS SCALE supports HW pass through to app containers.
Virtualizing an OS can take a lot of RAM, and virtualizing a Minecraft server can take 8GB by itself if you use any mods - I would definitely recommend 16 or 32. The other concern is your storage - for a NAS you ideally want a RAID or ZFS system. You’d need at least 2 drives for this (for whats’ called “mirroring”) to give you some data safety, though with ZFS you can have an arbitrary number of drives and create storage pools with arbitrary levels of redundancy (this may be more than you need though, and would require more RAM as well)
Quick, cheap hits:
As someone who deals with this both at home and at work, 10+ years ago there were valid reasons to not use virtualization, but these days there really aren’t – especially at home, unless you enjoy the pain of building things back up from scratch (on your own time) when something breaks. Disk IO is good, GPU passthrough works, USB passthrough works, etc., etc. Maybe very, very edge case scenarios like mining chia or something would still benefit from bare-metal access, but that’s not what we self-host for, right? ;)
This isn’t a limitation of TrueNAS, but rather of ZFS itself. A new ZFS feature that allows expanding the pool one drive at a time has been in the works for a long time now and is supposed to be rolling out really soon (as it has been for the past 2-3 years I think), but as of right now it can’t be done on any NAS solution that uses ZFS.
As you noted, UnRAID can do it - because it uses a different kind of data+parity system (you can continue to add a drive at a time as long as the largest drive has your parity data, I think). If you were comfortable running TrueNAS in a VM then you should have no problem running UnRAID the same way. In fact, it will probably be faster just because it’s less resource intensive (and certainly less RAM intensive than ZFS)