> My strong preference is a key command for switching from Bluetooth/USB, and that the keyboard keep both active for instant switching. The only keyboard I’ve found that does this well is the micropython-based M60.

Epomaker's keyboards also do this, at least my SK66 does. Fn+Space switches between wired/BT and Fn+[Z|X|C] switches between the three BT connections. It also flashes blue on the active connection so that you know you've switched into BT mode.