If you just want your Raspberry Pi to stream music to a dock or powered speaker, all you really need is a DAC hat (cheap & good; Audio DAC SHIM from here ThePiHut) and install HIFIBerryOS on the pi itself. Honestly, such a simple, reliable system, I’m shocked I didn’t find out about it when searching!
I spent months faffing about with different distros and music specific eco-systems just to get Spotify Connect working, and kept hitting brick walls and problems.
Volumio, Mopidy, RaspianOS with Spotify (kinda) installed – they all had issues and essentially wouldn’t output sound or had a monthly sub fee, which sucks.
So, gave up in the end and had a Pi and DAC doing nothing, until I found HiFiBerry OS
It’s a really small image (about 400mb), you access it entirely from a browser, so no need to have a screen, keyboard and mouse attached to the Pi.
Setup is simple, connect your phone/laptop to the WIFI hotspot created on first boot, open a browser and type it’s IP into the address bar (get that from your broadband hub) and you’re ready to configure.
Install the Spotify add-in (made easy from the menu) and add your Spotify Pro login details (yes, you need Pro to use Spotify Connect) and that’s it.
Back on the phone/laptop, load Spotify, start playing some music and tap the little “Devices” icon, choose HIFI Berry and hit Connect. Job Done !!
As a side bonus, it’s all ready to play tunes over Bluetooth – simply connect phone/tablet to HIFIBerry (BT device) and play music from any app, Tidal, Amazon.