I kind of get what the OP is trying to do here, he’s not bothered about perfect stereo imaging etc, but just wants to fill the room with sound without having to faff around with volume adjustments all the time via normal grouping of speakers.
Unfortunately there is no way in the Sonos App to permanently ‘bond’ a set of speakers that don’t fall into either a stereo pair or home theatre setup.
You can create a pre-defined group (under manage system) which appears in the top of the screen when you select a speaker to group, but if you have other speakers it doesn’t appear to work correctly and just adds the group to the currently selected speaker. (Unless you hit everywhere first followed by your group...)
There is an app (costs money) that may do what you want called ‘Soro - for Sonos’ on Apple products which used to have the facility to preset volume for a group of speakers.
It’s something that used to be in the Sonos App once upon a long ago and got lost in the drive for ‘improvement’. Perhaps @Corry P could mark it as a feature request?