a much simpler solution is creating a http endpoint that exposes the relay's internal event primary keys (serial numbers) and then syncing is just keeping a count of the last newest event serial they have already got. et voila.

i got the idea from nostr:npub12262qa4uhw7u8gdwlgmntqtv7aye8vdcmvszkqwgs0zchel6mz7s6cgrkj (bless him) and it's something i will be adding back to my relay in the near future but any relay dev can easily implement this shit. it's retardedly simpler than negentropy.