TrueNAS Scale is a good option. ZFS is a very resilient filesystem. I lost a lot of data to a software raid in the past that didn’t checksum the data and now I have an affinity for zfs. I believe they have added the ability to grow with larger drives as well - just disconnect drive an and insert new larger drive b, let it resilver, and once you’ve got them all replaced it grows the volume. Set it up, see how you like it, and move your data over if you do.
You may be different, but given that your current situation is a couple drives sitting on a desk for 4+ years, I wouldn’t worry about expansion so much. I built a nas a while ago and figured I’d upgrade it, and I haven’t. Until it’s full, it’ll keep going.
Also check price/gb before settling on 6TB. That’s small.
It’s pretty good to work with, and it’s got pretty mainstream support because the OS isn’t FreeBSD anymore, and it supports docker. As far as setting up the array you plug in the disks and tell it to make a pool. Pretty easy. Then you can subdivide as needed.
TrueNAS has some built in support for backing up to various clouds via rsync, or you can sync at the pool level to a remote server.