Soooo yeah. I understand brake wiring is stupid simple, nothing fancy or complex that is needed. Went down through the underbelly and checked every possible ground there, nothing bad, funky, loose, etc. Removed each ground I found, cleaned it up and re-secured it, including removing the main ground within the junction box.
12v testing throughout didn't show any crazy readings. Just absolutely MINDBLOWING what happened. I'm lost on this - this calls for someone wayyy more skilled than I am with electrical.
Played trailer tetris again today - sure enough all the way TO the yard I had brakes all the way there.
I'm starting to NOT think that the trailer brakes is the root cause of my issues, but something else in the 12v system, a short, ground, etc. Something, somewhere.