On Android and IOS, i like Snikket.

On desktop, Gajim

For older/lower end computers, enhanced-h264ify is a godsend. It blocks youtube from playing VP9 codec which can cause heavy cpu usage on older hardware. It can also block 60fps video as well.

I usually name mine after songs that I happen to be listening to at the moment.

Ranger, mostly for bulk renaming

A mouse is a must if you get a dock for it. Some emulator settings are impossible to get to with an external controller. Keyboard is also nice to have.

SD card obiously for more storage.

I use it for seeing/replying to texts on my desktop. It works great for that with xfce.

I don't have any experience with it, but there is a project called mopidy that I believe can play from many different services, including local music and spotify.

I would get a powered USB hub for your hard drive, then plug in the hub into your RPI4. The usb ports on the pi cannot handle a lot of power(from what I remember).

Jellyfin, I've run mine for a couple years now(ever since the pi4 came out) and it's been great. The only thing to consider is turning off video transcoding, its not able to to transcode anything HD or over. Other than that, its great. I use it for videos and music.

I didn't have one, but my friend did. We played a lot of Star Wars Battlefront II.

Organic Maps. It uses OpenStreetMap as a backend, plus it has hiking trails as well.