This is amazing! For many years, I've been a happy customer (no other affiliation) of Rogue Amoeba, whose (closed-source, commercial) Airfoil software: https://rogueamoeba.com/airfoil/mac/ does all this & more (MacOS and iOS only). I've had a multi room audio setup with Airport Expresses and dumb powered M-Audio bookshelf speakers for the past 10 years going strong!

I love this project, but for folks who use MacOS and want something that simply works, I can't recommend Airfoil highly enough.