As another data point, I installed the 6.0.5 driver (on MacOS 11.4) and two SoftRAID volumes would not mount. DiskWarrior didn't see them. The log fil...

Since there seems to be no chance of recovery, I deleted the volume and recreated a new one with the same disks. So far after a few days it is working...

Assuming this is not recoverable, which is seeming more likely than ever, can I reformat the drives and start over? Or should I get new drives? I have...

Yes. From my original post: "I also tried connecting the disk chassis to my 2015 MacBook Pro with the latest version of MacOS Catalina (10.15.6?). It ...

Files attached.

All four drives show zero I/O errors and SMART status of PASS. No predicted drive failure message.

Followed your suggestion. Did not help. Also, I have a ThunderBay Mini with 4 SSD in RAID 6. Still works fine - same iMac.