On the Synology NAS, are you running DSM 7 or 6?
I’ve been seeing a LOT of SMB share drops on my Mac Mini M1 (macOS 11.4) and Synology D420+ (DSM 7) recently. Whenever I start to do some major file activity, the volume just drops.
I have Plex running directly on the NAS, so it’s not a Plex issue.