The best anti-virus is your brain.

Is there any application that lets you manually add metadata such as album art and co into files?

I’m surprised no one recommended syncthing.

Syncthing lets you sync changes on any folder/drive across multiple devices via the local network - no cloud needed. I currently use it for my keepass database, Music folder and Documents folder. It’s als very simple to set up.

Only downside to this is that if your house burns down you’ll lose everything - but a friend suggested me to have important files on an encrypted tarball stored in the cloud.