Actually you can do that kind of routing. Ableton works a bit like Reaper where your audio tracks can also be Bus/Aux Tracks. In Ableton, you create an Audio Track, Route all the audio tracks you want (say all drum tracks) to that audio track. Send that audio track to the master output. Rename that audio track to "drum bus.". There are also send tracks too (mainly for reverbs and delays).
Ableton can do any routing logic does. Actually if you have hardware synths and/or Eurorack, Ableton is perfect for you. There are Midi to CV, Audio to CV, and CV timing tools. There are Max for Live apps that specifically control your Eurorack. Ableton is perfect for that.
I couldn't get on with Ableton for 2 reasons:
The live audio engine is optimized for live performance. So it uses more CPU than more traditional DAWs because it's really build for using live. Ableton live is a bit of a CPU hog if you have projects with large track counts.
Ableton's workflow is geared for live performance. I am used to linear workflow DAWs (Cubase, ProTools, Logic Pro, Studio One) so the workflow drove me crazy.
What I did like about it was it's great for songwriting. You can write ideas very quickly in Ableton. I just could never finish. I may eventually get Ableton Live lite specifically for songwriting.
Back on topic. The sad thing about this sidechain PDC issue is the "rules of engagement.". Do this it it works. Route this to an Aux track it may work but run and Aux to an Aux that may break it. Having to figure out the rules must hurt workflow too. I saw a Youtube video with Disclosure where they specifically use ghost trigger tracks to use for sidechain in Logic Pro. That may be the best option.
This PDC issue is very frustrating. I can't believe you guys have been dealing with this for 4+ years!