Any desktop computer that has enough SATA ports and space in the case for your desired configuration, that is from this last decade, should work fine.

Dell rackmount servers are power hungry and loud.

TLDR:is a 4 Bay chassis provide sufficient protection for freenas ZFS with total ~ 10TB usable storage? or do I need 8 Bay?

There is no requirement of a particular number of disks for ZFS. You can run ZFS on a single disk.

But for TRUENAS (FreeNAS name has been dead for years now), you will dedicate one entire disk to the OS, so keep it small (just shove a small SSD in the case somewhere), and then decide how many disks you need to buy to accommodate the amount of storage you need. Then decide if you ever want to expand that capacity. THAT should determine what kind of case you buy.

Is a dell R320 or R510 sensible in terms of power efficiency for freenas?

Dear God, no.

I've used it. More just so I had a "landing page" on my NAS and pretend what it would be like to have a web GUI on a NAS

It's OK. It doesn't handle RAID, ZFS, or anything other than just a MergerFS setup or single disks.

I like the built in file browser, as it offers SMB sharing with just a right click on a folder. Handy for those times where you just need a quick SMB share for a bit.

Should I be worried about sensitive data being leaked?

If you're not exposing any services over the internet...no?

Can you elaborate on this a bit more because I'm trying to figure out where you would arrive at that conclusion that it would be possible. Perhaps there's something in your setup that you haven't explained fully.

Could it be as simple as grabbing a docker image and running the VM inside a container?

That's not how docker or VMs work.