"DevonMasterData.dtCloud" - GIANT file that is pretty old; do I need it?

.dtCloud is a DEVONthink (local?) “Sync Store”. 336.8 GB indeed seems very big compared to your database. But keep in mind that sync locations/sync stores can be used to sync multiple databases. Are you still using Dropbox for sync?

Maybe it would help to check the manuals section on sync: In & Out > Sync

Also, if you search the forum for .dtCloud Dropbox, these are the first 2 results: