I also would recommend to do the switching on the MIDI NoteOn side, instead of bypassing the VST instruments.
My solution is to use widgets to directly block NoteOn (!) messages of those MIDI In blocks of splits and layers which should not sound at the moment.
Then you could use variations or songs parts to quickly switch between different splits and layers on all of your keyboards.
The solution of @Vindes is in fact quite similar with same result, at the expense of addidional blocks, for sake of a better overview.
See also this thread:
It seems that @rtwfroody is asking something quite similar.