Alright then. Turns out that what I actually had in mind is not possible with the Nextcloud mobile app as it cannot be connected to any arbitrary folder that I defined. Instead, the Nextcloud mobile app will use some self-defined folder on the mobile phone’s internal storage and this way, will clutter it with my 55G of music.
To clean it up, go to the /music/ folder in Nextcloud mobile app, tap the three dots again and choose delete. There will be a prompt so you can delete just the local stuff on your mobile phone, not the entire folder in the cloud.
The real solution is an external app FolderSync (you can also go for the pro version). FolderSync is able to connect to your Nextcloud server with your credentials. You simply create a pair of folders to be synced, adjust time and options and off you go.
Good luck!
[edit] Found this solution here: Gewissheit ĂĽber syncronistion mit android (German)