Huzzah! I think I’ve sussed it - Group Channel Tracks!
It is more fiddly to set up, but does come with the added bonus that I can now fully automate much of my stem creation. I basically create a Group Channel Track and treat it just as if it were a bus - pointing all the Group Channel Tracks to the Main Mix Bus. With the ability to drag and drop inserts etc in the mix window it shouldn’t be too much hassle to import 6.5 projects.
For an Orchestral template, the added bonus is that you can send several group channel tracks to another one. For example, I can now create a group channel for high woodwinds and one for low woodwinds but send them both to a woodwinds channel for easier monitoring of say woodwinds against strings. Then when it comes time for a batch mixdown, I can select the hi & lo woodwinds separately (no more mute all, solo, mute the solo, solo some more mutes - coz if you do it the wrong way round they stay muted anyway - malarkey!).
I still think you should be able to route any bus to the master mix - otherwise, what value do busses add?, you may as well not have them if you can’t monitor the sum of their outputs - but for the meantime, I don’t have to switch back to 6.5, which will do for me!
Now if only there was a way to link specific midi tracks to group channels so they had linked solo & mute capability I could automate all of my stem creation …