Don’t commit too quickly. btrfs has it’s advantages, but it has limitations too - not least of which is that there’s no two-drive redundancy mode, and the raid5 mode has some known bugs - it’s mostly safe. Generally I’d go with ZFS, as would most here.

Certainly some variation of linux, but there are many of those. For drive pooling, your options are btrfs or zfs. Of those, zfs is the most mature and capable - the only drawback is that once you make a pool you can’t easily change the number of drives in it. A small limitation.

Drive pooling as ZFS does it is a replacement for RAID. You generally don’t need RAID these days, except for fault-tolerant boot drives in high-availability servers.

Ignore the ZFS “1GB per TB” thing. It’s old advice. Use, ZFS does like lots-o-ram because of the way it uses caching. But it doesn’t need that much. Your 16GB plan is plenty. You could probably do it in 8GB.