Risky

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.

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.