Is setting the HT surrounds to use ‘FULL’ (range) stereo playback for music audio not working for you in this instance, as mentioned here…
https://support.sonos.com/en-us/article/low-audio-from-surround-speakers-when-listening-to-music
In my own use-case, I don’t like the music soundstage behind the listening position, but each to their own, I guess.
It’s important to understand what is at work here. When adding surrounds/Subs, in order to maintain sync with the video, the radios are reconfigured to a low latency, one-way dedicated 5 GHz connection from the soundbar to the surrounds and Subs. This requires a series of handshakes to get right, which is why adding surrounds/Subs takes a bit of time. The reverse happens when the surrounds/Subs are removed. So therefore, a quick toggle between the two is not possible. They could place a hot button that does the add/remove at the touch of the button, but that is not going to speed up the actual process, and doesn’t save much from the system menu commands we have today.
This function already exists. Just set Music Playback to Full.

This function already exists. Just set Music Playback to Full.

I believe the OP is asking that the front speakers/soundbar be taken out of the picture and just have the rears playing stereo.