This gives me an idea;

Don't store incoming data from remote instances into the "Main DB" immediately. Store them into SUBORDINATE DATABASES!

The logic of how you arrange these subordinate databases should be simple; depending on which instance you're communicating with you could select a subordinate database like so;

• First; we need to have a "Main Delay" database. This database is used by all the instances we Both Federate With, and Mark as one we Trust! and we merge all records here into the main database on a specified timeframe to give ourselves a little time to roll back the clock if something betrays that trusted status.
• Secondly we need to have unique little databases for each little instance that we Federate with, but do not yet mark with trust! These little DBs are merged into "Main Delay", then Main on a different time-delay schedule. This gives us even more time to roll back large-scale attacks, spam or flooding via ActivityPub as well as time to just smack the "Defederate" button as soon as they start to misbehave and, optionally, jettison the garbage data that caused the need for Defederation as well.