The current plan is 30V or perhaps 36V. It's difficult to find 5V buck regulator chips that accept higher than 40V input.
why not says that for input above 30V the 5V line has to be supplied via the ext 5V?
I think that the board could have separate V_MOT and V_IN terminals.
The V_MOT could accept higher voltage which would go solely to the motor drivers.
The V_IN could accept 25-30V max as you will probably need 12/24V for the hotends or fans anyway, or for other automation purposes in CNC. This is how I usually do in custom machines I design.
Those who don't need higher voltage or anything special just have to connect the V_MOT and V_IN parallel, those who need it could use two power supplies.