Okay, this makes sense! I'm not really storing sensitive documents at the moment, I'm just using RAID mostly because I'm dealing with second-hand drives and I'd prefer to have some peace of mind in the unlikely event that something fails before its time (which I understand is unlikely if I'm not doing frequent, intensive R/W operations, but still - I have a very cautious approach to storage in general).
That said - because nothing that's currently on the drives is irreplaceable, I could certainly see myself moving away from the current RAID configuration, and instead putting together a setup where I use the two 4TB drives for video, plus two of the 1TB drives for music (which has significantly smaller filesizes). I think I'd still like to use a basic RAID array (or, preferably, ZFS pool) for each group - primarily for the increased read speed, and secondarily because I prefer the flexibility of not having to mess around with multiple file paths and determine what fits where if the drives fill up. How does the following plan sound?
1. attach 4TB drive to fifth SATA port, make it its own *basic* ZFS pool, transfer all current data
2. shut down system, remove two of the 1TB drives, attach *both* 4TB drives in their place
3. add the second 4TB drive to a basic ZFS pool with the first, totalling 8TB of space
4. create a new, blank ZFS pool with the remaining two 1TB drives, totalling 2TB (for music)
5. optional: combine both pools into a single data folder with two subfolders
Thinking about step 3, though: would this require data to be redistributed across both drives, effectively resilvering and potentially wearing out the disks? (If it helps at all, this would only be done once; if I need to expand again, I'll save up for a pair of larger drives and use the same process described here.)
Also, if I end up storing documents and other sensitive data outside my main computer, I'll likely just save up for parts for a second NAS, and configure it with a mirrored RAID array of some kind. My ultimate goal is to divest from proprietary cloud storage entirely, while still maintaining a bit of flexibility for collaborating, sharing files, and working on-the-go.