Ok so I figured it out. The reason it was eluding me was that it was two separate things which led me to all the wrong conclusions. .
First setting inertia back to 0 (which makes it automatically compute inertia and clearly that's good enough)
Second one of the objects I was colliding with for testing had a strange wonky collision box. It was a slight rhombus although I saw nothing in its inspector to indicate why, and it was slightly scaled up. I am kind of amazed how such a seemingly tiny change had made that object into something that would teleport my tank to space every time I touched one of its corners (not in the video).
Kingstad
@Kingstad