I’ve had a great experience with the TrueNAS Mini-X system I bought. ZFS has great raid options, and TrueNAS makes managing a system really easy. You can get a box built & configured by them, with 16 GB ECC RAM and five (empty) drive bays, for about $1150 at the most affordable end. https://www.truenas.com/truenas-mini/

One thing to be careful about: you can’t add drives to a ZFS vdev once it’s been created, but you can add new vdevs to an existing pool. So, you can start with two mirrored drives, then add another two mirrored drives to that pool later.

(A vdev is a sub-unit of a ZFS storage pool, and you have to choose your RAID topology for each vdev and then compose those into a storage pool)

The common wisdom about backups is the 3-2-1 backup strategy, which recommends:

• 3 total copies of your data, including your original or “production” data
• 2 different forms of media
• 1 off-site copy

Proton Drive can be a decent off-site backup, but it would be a good idea to make a separate backup of your data on a different form of media like an external hard drive, just in case Proton Drive goes down, or the data there gets corrupted and you need to restore a known good version.