com.spotify.Client VS raspotify

There is nothing "unsafe" about flatpak. In fact for every package on flathub you can see exactly what the "unofficial" packagers do in the corresponding github repository. For example here is the repository for Spotify, if you look at the json manifest file you can see they are repackaging the "official" Snap with the necessary libraries and scripts to support it running as a flatpak. Everything there is open source and able to be examined.

If you click on the "Manifest" link for the package on flathub, it will take you to where the code that packages the app resides. There you can review exactly what the packager does in the json manifest. Here's the one for the Spotify client, for example, which shows where it pulls in dependencies and where it downloads the proprietary code from (in this case they base it on the snap) and what commands it executes while packaging the flatpak.

Try installing it via Flatpak

Flatpaks builds are made to be reproducible. Everything is done openly and you can check how the apps like for example Spotify where made.

Why not just use the flatpak?

Judging by the manifest, the Spotify Flatpak is built from the Spotify Snap.

sudo apt-get -y install curl && curl -sL https://dtcooper.github.io/raspotify/install.sh | sh

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.