I’ve post the MRM issue note in my original MRM thread. I still think the MRM should work for you, and the out-of-sync issue should matter when Alexa was asked to play music on individual device and MRM after a few times. So I am curious why it doesn’t work for you at all.
Unfortunately, grouping is fundamentally not supported by SmartThings capability model. However, to adjust volume across to different devices, you may consider to use the Number variable, and a rule to set volumes on the desired devices using this Number variable, when this variable value changes. So the variable value can be adjusted in the dashboard, and populated to multiple devices’ volume level.