Is there a way we can scale relational databases horizontally? RDS read replication is an option but that is only for reads.
For most databases the answer is no, but AWS has put in a bit of effort into their Aurora product to make multi-master databases a reality for the cloud: https://aws.amazon.com/about-aws/whats-new/2019/08/amazon-aurora-multimaster-now-generally-available/
This is pretty new as of writing this, I don’t think there’s a lot of experience out there with it. But you can always go to their documentation.