How big is the RAID set, as in, how much redundant space do you currently have, how much is used, and approximately how much do you add per year?
Do you need to keep all your data in one volume, or can you split it between old stuff and new stuff? You can do a set of 4 drive TB jbod arrays, or a single 8 drive TB array (Areca in jbod mode). Depending on what you do, how you access files, and how much internal SSD space you're purchasing, there are lots of ways to split this up. You can even go cheaper and do one USB3 (archive) and one TB (working set). It all comes down to what you want to spend.
How do you currently backup all these files?
Unfortunately there is no recommended take your disks out and put them into (box) and they'll just work. You will want to copy them to a new set, no matter the size.
Btrfs and ZFS are great, but depending on your technical aptitude they may not be in your wheelhouse.
-Joe