RAAT is a protocol used between Core, Remotes and Endpoints AFAIK and therefore doesn’t affect a NAS used as a storage device. Roon makes use of the SMB protocol to access external storage. See also:
Best solution IMHO is to use local storage (internal or USB attached HDD/SSD) for the Roon Core and use the NAS to hold a backup of your library and the DB backups.
Else and if you really want to save power and hold the library on the NAS, shutdown the core when not in use. If it’s Nucleus(+) or ROCK based, press the power button to do so. If you run your own, you can use OS features to start/stop the Roon processes on a schedule also. It will then no longer access the NAS, the NAS can go to sleep – problem solved.