If I’ve understood your request correctly–I don’t understand the reference to Spotify since Roon doesn’t support this service–this feature is already available under Versions. Simply make sure your Library version is the Primary Version.
Ok, let me rephrase that, my internet service is unreliable (because Frontier DSL sucks, avoid if at all possible).
I usually listen on the weekend and that’s when everyone on our circuit also seems to be using the internet. My service slows to a crawl.
It becomes a problem when Roon is trying to stream music instead of playing the music that is already on my hard drive.