As you mention, there can be lots of synths!
In my case, I can easily have 10+ different ones on one song, just because I like to work with synths that have a different color/tone and complement each other. Some are metallic/cold, some are more analogue-ish/warm, others have unique sound/filters, you get the idea… Having one bus for them wouldn’t help at all in my case.
Let’s not forget that most VST synths these days have incredible effects & mixing possibilities built in, so they don’t need that much processing (unlike an electric guitar which you may have to go through 5 different pedals or other effects to achieve the sound you want).
As a musician, I am a guitar & bass player first - yet I never use analogue instruments in electronic music. Maybe I’ll get some guitars back in there at some point; don’t really feel the need right now.
Since you’re planning on mixing acoustic instruments with digital, it reminded me of this thread you may find interesting: