Well, as above if you are running MC installations as independent DLNA Renderers, and not MC Server Clients, then Volume Leveling is available under the DLNA settings for Audio > Advanced. But I haven't tried it and don't know if that actually works.
An Id running in the garage, with wired speakers, might be the way to go. But I'm not sure if it would sync properly to a Linux based Id, so a NUC running Windows and MC would probably be safer. Then you could use Bluetooth speakers, since the NUC and Windows support Bluetooth.
You may even be able to play to JRiver for Android on an old Android phone at some stage. But at the moment playing to JRiver for Android from another MC installation is just too unreliable.
You could then control the server for playback using any of the appropriate remotes (Gizmo, JRemote, MO 4Media) or Panel from a phone, all over WiFi, Playing to a Zone Group (I think that works) that includes all the Zones (MC installations running as DLNA Renderers) you want to play in sync.
But I haven't tried this, as I don't have enough Windows-based devices to test it properly. If you are willing to invest, it could be worth a try though.