I've researched this quite a bit in my quest to find out what is the 'modern' way to do whole home audio that isn't Sonos and isn't tied to one expensive specific brand which would likely stop support eventually. I've come to 2 conclusions.
1. Get a 'dumb' 6 zone amplifier like THIS
Monoprice amp and put a Google Chromecast, Alexa, or similar device on each zone. With the Chromecast audio (discontinued) you can then setup groups such as 'Whole House' or 'First Floor', etc.. and just say "OK Google, play Spotify on the 1st floor". There are zero physical keypads or volume controls. The big downside is you can't mix and match inputs and zones which is a big part of WHA. So I can't have a TV input that is playing on zone 1&2 and a Chromecast input on 3&4, and bluetooth on 5&6. I originally was going this route, went as far as buying the amp, chromecasts, and ethernet adapters. I switched to route #2
because I wanted the ability to have many different input types and also wanted keypads as well as phone control as I didn't want to always be forced to use my phone. Also, I've found that the Chromecast audios are finicky at times and my phone will lose connection with them which takes a bit of time to reconnect to them.
2. Get THIS
Monoprice 6-zone controller and amplifier. Wire up keypads (if you prefer) so then you have full physical control at each zone. To get phone control you can do a quick search on this forum and find out how to use the RS232 serial input to control the amplifier. There is a bit of setup required to make this work, but really isn't too complex if you're somewhat tech savvy. Basically involves buying an RS323 to ethernet adapter, setting up a bit of code from Github on a rasberryPi or some other home server, and purchasing a $5 phone app.
Now if you are really tech savvy, you can learn how that API works and write your own code to control it. The reason to do this is you could then tie it into a central home automation controller such as HomeSeer and build out a custom UI using HS3Touch Designer to control the amplifier from a wall tablet, PC, phone, etc. Could even go as far as buying some Lutron Pico remotes which can be programmed through HomeSeer to control the specific functions of the amplifier and you can mount these in the wall (here's your wireless volume control), on a stand or use as stand-alone controllers. Many options open up if you go down this path, just involves a lot more tinkering.