Multiple DB instance sync up

Hi I understand you are trying to find a best way to sync two or more multiple instance.

Instance per instance syncing you need to setup logical replication (one way) or physical replication (read replicas).

well, you can sync database in RDS instance by implementing replication[1], remember that an RDS instance can have up to 5 read replicas. and if you setup your own logical replication you can have more, but for logical replication, it depends on storage available,CPU, memory etc.

Reference link

1.https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_ReadRepl.html