You can definitely use the Studio Sends as alternate mix busses.
Here’s what I used to do:
I have a MIX group that gets everything, and it sends (pre-fader, 0 dB) to alternate MIX groups (MIX1, MIX2, etc.). Those outputs are set to “No Bus” and they each send to to their own Studio Send (MIX1 to S1, etc.). Then in the CR mixer you can switch your monitor source between MIX and S1-4 to flip instantly. And you can set up a key command for Switch: Source Select.
That’s fine if you don’t use the Studio Sends for monitoring during tracking. Since I do, I now use the External Ins in the CR mixer. Same setup with MIX1, etc. above, but instead of sending to Studio Sends and routing to “No Bus”, I route to hardware output, an RME MADI card. The RME let’s me loop back hardware outputs to inputs, so I have MIX1 going out ch 49-50, looping back and appearing on inputs 49-50. And so forth for MIX2. Then in the CR mixer, I set CR Source to EXT and switch between external inputs.
If your I/O hardware won’t loop back internally, you can always patch outputs to inputs.
As always, your mileage may vary! By the way, this is all easier in Nuendo using Direct Routing.