The buffer on the system disk is only for content fetched over the internet. It’s there to hedge against irregularities in the internet connection, and to smooth over some details about how the streaming services actually deliver content to us which would otherwise cause user experience harm. Think of it more like a browser cache than a memory play buffer. It caches the un-decoded file bytes right next to the HTTP layer. Content that is already on the local network does not go through that subsystem.
Putting it in RAM would not help it achieve its goals, and would work against the health of the system as a whole by putting extra memory/performance pressure on the machine running the Roon Core. I don’t see any “better” there.