I finally seem to have fixed this issue! It had NOTHING to do with hardware after all that. I found an obscure Reddit comment from a year ago that mentioned Docker consistently crashing and shares becoming unresponsive after a couple of days. The user worked out that there was a bug in either FuseFS, Docker or Unraid (or a combination) where there is a chance for FuseFS to crash when a Docker container uses a FuseFS share (usually /mnt/user/appdata). Switching all my containers to use /mnt/cache/appdata fixed the problem and they no longer crashed.
I found an option in Unraid under Global Share Settings that enables "exclusive shares" so as a test I set my containers back to using /mnt/user/appdata and sure enough, after 2 days, it crashed. I restarted, enabled the exclusive shares option (and made sure to change nothing else) and it's now been 8 days (the longest uptime I've had in over a year) since my last crash, seemingly confirming it's an issue with FuseFS, Docker and Unraid.
Unfortunately neither myself or the Reddit poster have been able to find or properly report the issue as it's not consistently reproducible and doesn't generate any logs.
