It’s probably RAID 1, two disk mirror. As far as I know only QNAP Hero devices support ZFS, and ZFS is a lot more demanding on the system.
Expanding your system would only be possible via USB enclosure, not TL-D800S, that one connects via 2 x SFF-8088 interfaces. You can check compatible enclosures on their website, but those tend to cost a lot for what they are.
Since you would still need new hard drives, more RAM and relatively expensive enclosure I would personally opt in for a new NAS and use the old one as a backup target for it. Maybe even take it offsite.
If the new one is QNAP you can (probably) replicate snaphot shared folders to the old one automatically with very little setup.
That’s certainly a convenience factor. But if you can build a PC and download an ISO, that’s pretty much what it takes. Compatibility is really good for Core and probably even better with Scale having Linux Kernel.
If you’re going for >4 drive bays, you’ll also save a lot compared to the TrueNAS mini line, which is kind of dated by now when it comes to hardware. I like the Supermicro cases they use, but sadly Supermicro doesn’t sell them anymore.