Hey Grady, great question.

I've actually created a video guide for just this setup here.

Thats a great strategy to keep RDS as fully locked down as you can. The nice thing about this whole setup is that the containers on Cycle will automatically understand how to communicate using the peering connection and out of band networks to reach the RDS instance. This works both in the same region as the RDS VPC and from other regions as well (multi region).