to do separetad mixes that are separetad in all aspects you need a real broadcast mixer with more than one mains or two completly seoparated mixers
or… tada a dLive
the dLive is able to handle up to four separated workplaces with separated mixes and solos in one S-class mixrack
to handle it like the pro’s are dooing
something like the Lawos or the Stagetec consoles are needed
If you have the channel space available you could split all the channels you want to independently mute only in Aux 1.
One side of the split would be for mains and any other Auxes.
The other side would be dedicated to your Aux 1 feed.
You would also have the added benefit of independent channel processing available for your Aux 1 feed.
The split would take place in the input routing, you could have a layer of
just the channels dedicated to the Aux 1 feed.
Yes you could apply an processor to any channel and have it only return to Aux 1.
Just think of the FX returns like you would any input channel.