Instead of using a thumb shift, I was given the idea of using the pinky keys as dual-role keys that also send the Shift keypress.
This gives me a Shift key on each hand again, but it will make me learn to Shift with opposite hands after all.
Atreus: Removed home row Shift. It's under the thumb anyway. Also replaced dual modifier keys with macro keys (cut, copy, paste, undo).
Ergodox: Made Colemak the default layer instead of Dvorak. Also began the process of bringing it in line with the Atreus layout I've been working on.
Joshua T