I've been using linux for a long time, and I have a background in this kind of stuff, but it's not my career and I don't keep as current as if it was, so i'm going to give my point of view on this.

A zfs array is probably the legit way to go. But there's a huge caveat there. If you're not working with this technology all the time, it's really not more robust or reliable for you. If you have a failure in several years, you don't want to rely on the fact that you set it up appropriately years ago, and you don't want to have to relearn it all just to recover your data.

Mergerfs is basically just files on a bunch of disks. Each disk has the same directory structure and your files just exist in one of those directories on a single disk, and your mergerfs volume shows you all files on all disks in that directory. There are finer points of administration, but the bottom line is you don't need to know a lot, or interact with mergerfs at all, to move all those files somewhere else. Just copy from each disk to a new drive and you have it all.

Snapraid is just a snapshot. You can use it to recover your data if a drive fails. The commands are pretty simple, and relearning that isn't going to be too hard several years down the road.

The best way isn't always the best if you know you're not going to keep current with the technology.