Hey Eric, for a point of reference, approximately how large are the photo directories for the last few years - aka how much are you adding each year?
I'm reading your disk layout as:
TB2 enclosure: [8tb Active] [8tb Backup1] [8tb Backup2] [empty]
I think you're close, but there seems to be an issue or two. The platter hard drives (8tb) should be noticeably slower than the internal SSD, or a SSD in the OWC JBOD if it's connected by TB2. RAID4/5 should be a bit faster than a single drive, just in that there are 3x the drive writes for 1.5x the data (slice A, slice B & Parity). Just to confirm when you save to the internal drive of the Mac Pro it's crazy fast (relative to saving to the external drive).
The more I'm thinking about it there is a few options to consider. You can upgrade your 4 disks to 12tb's for around $1,600 and gain some 3tb of free space, and maintain your current workflow and methods.
If you were to add disks, I'd actually think about snagging a "OWC Mercury Elite Pro Quad RAID" as an external usb3 enclosure that we'd move or add disks to. TB2 is great, but it has a price premium, and for backup disks to be in a separate enclosure adds another level of protection (enclosure going bad). I'm thinking of adding 4x 8tb drives, so you'd be gaining around 7tb of usable space. I would also add in a 1-2tb SATA SSD to the shopping cart and configure it like:
TB2 enclosure
[New8tb Active] [New8tb Backup1] [Old8tb Active] [NewSSD]
USB3 enclosure
[New8tb Backup2] [Old8tb Backup1] [Old8tb Backup2] [empty]
4th new 8tb is the second offsite disk.
I'd most likely mix the new and old disks - as in have all data on new disks & old disks. This would give you fast access to the new & old data, the initial backup of new data would be within the TB2 enclosure, and all data would live on both enclosures.
The SSD would be on the faster bus & would allow for better saving performance. I'd check which SSD you tested with as some of the older ones that need to 'wipe' a cell before writing to it (that whole TRIM thing) may have impacted it. OWC sells the enclosure with and without SoftRAID XT, it adds $70 to the price.