The “best content”:

You mean like Matrix ?

Someone who hates Mike Patton can only be a POS, so no surprise here

Thanks but the goal is to sync with the “cloud”, for backup in case of fire or something.

I have hundreds (thousand ?) of albums I need to backup. I can reencode them cause 98% are on CD, but if I loose both my computers and my CDs, I’m done :/

I only use cloud backup for music and the few photos I take with my phone so I don’t really need real-time syncing.

Is that the proprietary FolderSync or is there a FOSS solution for syncing folders ?

I used FolderSync with OneDrive (in the past) a’d it worked ok, not shitting on it, I’m just looking for a FOSS equivalent with Nextcloud