First, read the instructions for how to setup Bonjour in the “DEVONthink Manual”, supplemented by Bonjour Simplified
Turn off your iCloud sync setup by unchecking in the “Locations” tab in Sync Settings. Setup and use Bonjour.
PS. it is unlikely your iCloud sync location has “ballooned” unnecessarily. And if you have insufficient space on iCloud server for synching, that also could be getting in your way. Dunno. The sync location contains the data that DEVONthink needs to sync. It is not a DEVONthink database. You can “clean” (delete) it and start again if you want. “Cleaning” also discussed and explained in the “DEVONthink Manual”.
PPS. DEVONthink “Help” has same content of “DEVONthink Manual”