You are probably not doing anything wrong. This is the last big thing, I am investigating and trying to fix.
On DSM6 Roon Server had root privileges and was able to increase the resource limit with the ulimit command. This is not possible in DSM7 anymore, as root privileges are not allowed for third-party apps. I am hoping to find a solution for this. Until then you have to hit the “Force Refresh” button in the storage settings.