[baumy]

To directly answer your question - when I was trying to setup whole home audio and decided I didn't want to go the rpi route of doing it myself, the alternatives to sonos that I found were "Audio Pro" and "Bluesound". I ended up going with Audio Pro, so can't speak to bluesound, but it should tick all of your boxes.

I have 3 Audio Pro speakers, 2 different models, and they all sync up perfectly. I can select 2 to be stereo if I want. I can play from spotify, airplay, bluetooth, or google cast. The models I have do support 3.5mm input, but I've never used it so not sure if you can plug into one and have it also play in the others. As a bonus, it's all compatible with the wiim streamer devices. After buying a couple wiim minis, my klipsch TV speakers + computer speakers are able to join the whole home system with the Audio Pro speakers.

If I had to do it all again, I might just buy powered speakers of any brand of my choosing and get wiim minis for each of them. That's another viable alternative you could look into. But the Audio Pro speakers are pretty nice imo.