Mopidy MusicBox VS raspotify

Is there a reason you need to use carplay instead of something like pi musicbox? What exactly is the USB dongle for?

I saw something here on reddit where people were taking the little $5 raspberry pi zero's and basically building their own chromecast audios with them. I am not sure this is the same thing I saw but I found this on google. https://www.pimusicbox.com/

I have a few rooms in my house which I have audio, but they are far enough apart that I won't be trying to stream to all at once. I am also a mostly Apple user, so I use Raspberry Pi's with Hifiberry AMP2 and shairport-sync (AirPlay target). I can select any of the Pi's as the speaker output on any Apple device. There's also a pre-made Raspberry Pi distribution called pi music box which also offers other protocols than AirPlay.

Get yourself a Raspberry Pi and battery pack if you're mobile, and check this guide out. That'll run Spotify 24x7.

Look what I've found: https://www.pimusicbox.com/ This should perfectly suit your needs..

Currently, my plan is to use https://www.pimusicbox.com/ with some sort of DAC, an undetermined raspberry pi model, and a micro SD card for storage (total flac library is just under 500 gigs). I'm not incredibly well versed in pi projects (have made a few successful retropie builds for friends) and am an amateur when it comes to coding and linux. Regardless, this project will be a hobby , so learning, failing, and growing are all part of the plan.

Pi MusicBox

https://www.pimusicbox.com/ I found this which has chromecast support and airplay, but it doesn't support rpi4 it seems

Basically Raspotify but for windows.

If you have a raspberry pi, Raspotify works pretty great as a Connect receiver with a little tinkering.

Raspotify installed on a raspberry pi would be my recommendation. Config allows you to normalise volume controls - have a read here: https://github.com/dtcooper/raspotify/wiki/Configuration

The last.fm site will work independently from airplay, so it will display whatever is playing, even if you are not using airplay to stream to your pi. Because it is just a standard Raspberry Pi OS running, you could also set it up as a MPD client https://linuxhint.com/raspberry-pi-music-station-mpd/ or https://github.com/dtcooper/raspotify. As long as you are submitting music to last.fm the song will display.

I have previously used raspotify on my 3B+, is there any way to achieve something similar on a pico?

For Spotify Connect on a Pi I can recommend Raspotify:

https://github.com/dtcooper/raspotify

https://github.com/dtcooper/raspotify allows you to use Spotify connect on Raspberry Pi's, so you can manually move your music using the Spotify app.