The End
So after some exhaustive testing, trying any and everything I could think of over the last few days. I didn't see, or capture and likely will never know or capture what triggered the meltdown or why, just the when it started, the how and what got impacted or corrupted, but not the triggering event.
What could possiblly cause a partial HW reset to occur contiously over 12 hours wiping the Router config, jffs, USB, nvram and partially and restoring it continously over that time, corrupting and confusing everything and every device along the way?
Well, I've tried heat (removed fan for a few days, but never get warm enough to even be an issue or concern), Power didn't blink or even brown out (other devices would've complained as well and none did), swapped power bricks between the AX86u's and the AX88u (round robin to cycles them through each over a few days) and nothing. Wasn't the USB as I'm actively using it as my secondary target for BACKUPMON and since I used FAT, I use the Heatlh Scanner via the GUI to check it (at first daily, now weekly not at the same time as BACKUPMON uses it as it disables writes when it runs). Router and Nodes memory passed every run of the mtd_check tool, with no or just a couple of bad blocks on one node.
Then I checked if maybe something shifted physically and caused the HW reset, continously over a 12Hr period, and back to the running config every few minutes. But nothing did and the Router and everything near it is in the same position as it has been and no physical contact where there shouldn't be. Not hacked, nothing exposed, nothing outside of the Router having a similar issue, every device exhaustively scanned and reviewed.
Long story short,

chalking it up to bad luck.
The positives from this being;
- Move to SSD.
- Enhancing my recovery posture, strengthening BACKUPMON (retention, and using a secondary target).
- Refreshing my config notes to have it at the ready if recovery is ever an issue.
- The documenting of the exercise of putting it all back together from scratch, eventually serving as the baseline when getting to a 3006 capable router and nodes with now compatible scripts.
- Getting over my want (purely want, not need) of supporting 6Ghz.
These are consumer level/based devices and I don't have the capability or could ever come close to justifying the time or expense to develope the capability with more extensive or a collect method of any more data that could possibly answer (but still not guaranteed) the "why" or if it would make any real difference if it did. Just adapt, overcome and accept what it is and do what you can to avoid the headaches if or when, it ever happens again.
Just happy to be running stable and a 100% functional again, The End
