ok now I understand what you are trying to do
the mixes are not independent
they all depend on the channel
but you are able to do a bit of independence
set the whole mix to pre fader, now you are nearly independent from the main mix
but now you have to control every part of that mix, the fx return too
if you send your fx post fader then you can bring down the channel with the send in the main mix and it automatic lowers the send level
but unfortunatly this is not possible for aux mixes since the send is just an aux mix with fixed destination only.