Thanks again to everyone for catching this issue and providing me tremendously helpful feedback!
Drilling out the pop-riveted snaps was a simple matter with a drill press.
I did some measuring, and about 12mm higher seemed good. Also, to get some slack on the belt so it would sit in front of the button plates, I eyeballed that I needed about 6-7mm on each side. Then I remeasured and punched:
Now, it looks like I’m at a better height, but there is some sag in front, as you can see by my thumb needing to hold everything up in this photo:
The back has the belt only on the kidney plate now:
Hoping this is OK now, perhaps with some more Velcro in strategic positions.