My Dream: Multi-room audio with Chromecast and central music server

I have been searching for a solution to my problem for a while now and I have had no luck - so i'm putting it out to the masses to see if anyone can help

The problem

I have some wonderful Chromecast Audios, which can be grouped and streamed to simultaneously. To do this, I can open Plex on my phone or iPad, and choose the sources and away I go. I have all my music hosted on a NAS, which Plex pulls from.

The "problem" is I have to carry my device with me to skip tracks or stop playback, if I move to the lounge from the kitchen but leave my phone behind, there is no way of stopping playback without finding the original device.

The Dream

What I'm after is an internal music server (like Runeaudio or Moodeaudio) running on a Raspberry Pi which is web accessible and can stream to a Chromecast. This means I can change the playlist or pause playback from any device with a browser.

Can you help?

If you know of anything suitable or can point me in the right direction, please get in touch as I would love to help/know!

Update 9/4/16:

The only way this currently seems possible is to install Squeezebox on a Raspberry Pi, install Chromecast plugin and access it through a myriad of apps. (Many thanks to Jim for the pointers!).

My main problem is the look. Squeezebox is ugly and even using Peng, it still is slow and complicated (although slowness might be down to the Pi)

I am still looking if you find a solution, let me know!

Update 4/5/17:

I seem to have accidentally fallen into a setup which works for me and ticks most of the boxes.

• Each room has a Chromecast Audio, which is grouped into a "Home" group using the Google Home app.
• I launch Spotify on my phone and selected the music/playlist required. You then cast to the "Home" group from Spotify.
• You can then log into Spotify anywhere and as you can only have it playing on one source at a time, you are able to skip/play/change music from any device with your Spotify details
• I've also recently got a FitBit Blaze which allows me to skip tracks (even when casting thanks to a recent iOS/Spotify update)

The only disadvantage this has is that it's not using any of the music I own - but since having Spotify i've not bought any music anyway!