Well written post and nice work on having the preliminaries done.
There are Bluetooth unit's that allow for remote speakers.
You'd buy the receiver, plug your speakers into it and any speakers become Bluetooth remote.
No wires and selectable like you wanted.
12 to 20 bucks on amazon and your phone or TV will also go to the speakers at your whim.
I don't know which one I have but it allowed me to not run a speaker wire from the projector, 30 ft across the floor to the speakers we use during movie night. Mine has a Bluetooth transmitter plugged into the aux jack and a Bluetooth receiver the speakers are hooked to. I'm all but certain it was under $20.
If your radio already has Bluetooth you're half way there....
Just a thought from been there, conquered that.
It's not what you asked but it's infinitely easier.
I'm all about alternative answers.