Music Player Daemon - mpd - is a small Linux utillity that does most of this.
It is a small music player runs as a service on a linux machine (RPi worked for me when testing) which can be controlled through a remote control app or desktop program.
It needs access to your music library, so look into sharing it, possibly through NFS, or set up a copy of your music library on local storage in the RPi.
I am a bit concerned about how well it would work with a shared solution, I know that some systems might lock open files preventing other clients from using them, but that is nothing I have tested.
My need for online photo galleries is just to direct friends and family to see them, I don’t store photos in the cloud.
I used to put up my photos on deviantart, I have had a gallery there for almost two decades, but lately dA has become very slow to navigate, so I built my own site, I didn’t need much, an index page linking to HTML galleries I export from digiKam.
After a few weeks of learning, designing and testing HTML and CSS, I have a nice index page that is responsive and easy to update and customize (in limited capacity).
This runs on a normal webhost, and is lightning quick to navigate, the galleries support browsing with the arrow keys, and just works.
There are three annoying things about it though…
To update a gallery, I need to recreate it in digiKam and upload it manually to the host.
I can’t include notes with the photos.
I have to edit a link in every gallery to make it be able to go back to the index page and not 404…