I'm posting this for anyone else who might have a similar issue. I figured out the solution in the process of writing the post. It was simple but not at all intuitive.
I recently finished troubleshooting a powersupply issue on my server that was causing random reboots, and now my union filesystem won't mount. I did have to run the system on a different CPU/mobo combo (both ryzen systems, same ram, and put both cpus on both Mobo at various points), and ran for a short time with all data drives disconnected. However, now that the hardware issue is fixed, and all HDD returned to the same exact SATA port as before, with the original mobo/cpu/ram, my union file system refuses to mount.
Each individual drive is properly mounting (using disk-by-label as before) and the system boots perfectly if I comment out the union file system line from /etc/fstab. at this point im not entirely sure how to proceed. If I try to regenerate the file system using the plug-in it makes the same line in fstab and then the system won't boot.
Is there any sort of clean up that should be performed on the individual drives to repair this issue? What logs or files should I provide here to help with troubleshooting?
While the system was running with the data drives removed (and thus the union file system unmounted) my docker containers had been running and one of them automatically generated folders in the /srv/[filesystem] mount point on my home drive. These folders were preventing the file system from mounting properly.
hopefully this helps someone avoid 2 weeks of troubleshooting for such a simple fix.