Is this just not possible or is there some kind of workaround?
Re: Logic... Stacks within stacks?
You can put summing stacks inside folder stacks, but you currently can't nest summing stacks inside summing stacks in the way you'd like, afaik.
You can of course move sub-tracks from one summing stack into another to make a bigger summing stack.
Indeed it seems as not possible at the time.
I like mixing to stems but the vocal and guitar ones especially gets pretty hefty at times and I thought I could organise things a bit easier if it could be done.