Advice on setting up Evoluent VerticalMouse


I have an Evoluent VMCRW VerticalMouse C, and with Big Sur came an end of using their custom control panel to set up the buttons. So I'm trying BTT (which I'm glad I found!).

Has anyone had any luck setting up the "thumb" buttons as modifiers? In the Evoluent control panel, I could set up the thumb buttons to modify the action of the main "finger" buttons when clicked.

The closest I've gotten is by mapping the mouse down and mouse up actions of the thumb buttons to key down CMD and key up CMD, thus using CMD as the modifier. But this is VERY unreliable. BTT often does not recognize that I let go of the thumb button.

Advice very welcome!!

Many thanks,