I guess one uses busses for various sub-mixes and selects from them in the export dialog, then?
That is how I currently do it yes.
If not, please explain how to go about preparing exports to various channel counts from one session.
However if/when a good surround ambisonics panner get implemented I would be doing mixdowns to listen to in multiple different spaces with different loudspeaker setups in each space. Thus it would depend primarily on the routing in the panner and the loudspeaker locations.
Seablade