mrvcoSwitcing Win(i.e. Cmd) with Alt is built into macOS. Just change the meanings of those keys in Keyboard Preferences. Look for the "modifier keys" button in the bottom right of the keyboard panel.
HoyleThanks, I've done that for a single machine, but it's not ideal if swapping between boards or machines. I got spoiled by my programmable Plum 84. I'll hold off and go with a HHKB as my first 'real' Topre board.