I like the idea of doing the RaidZ2 and a potential hot-spare. I think having one at the ready sounds like a good compromise on speed and safety, As for the usage, size, etc. The volume stores a mixture of data, primarily static data such as media (movies, pictures, etc), computer backups and VM disks. So while the vast majority of space usage is going to be large files, it's pretty well spread out. As far as the usage goes, quite low, It's mostly me and the other proxmox nodes that read from it. anything else is just writing backups now and then.
Admittely, as I've mentioned there is a lot I don't know about ZFS, My brief research about do's and don'ts mentioned these kind of things for performance. I don't think a read cache would matter as much as a mirrored write cache. As for that concern, I have two brand new Cyberpower 1500 AVR's on the way. I plan to link one of them to the NAS VM, TrueNAS has the ability to listen and react accordingly. I'm hoping this can help file corruption in the future, I had one on it previously, but it got old enough that I had to take it down, not long after, i found myself in this mess.
As for storage space in total. I think doing a single large RaidZ2 with a hot-spare will satisfy my needs. Worst case I can just make a whole new pool, no biggie. I've gotten about 20Tb off the volume so far, i'm now to the point of dealing with excess corruption in the XFS volume, Running the xfs_repair a time or two made some inaccessible areas reachable, but it seemed to always end with a "segmentation fault". I'm running it again now with a max memory set, and prefetching disabled, it seems to be doing something better this time around. I'll report back on that later. At this point I consider the recovery overall as a decent win. I got what matters most, but not all my hoarded data (yet).