By default sessions are either stored by Disk/Database. Since you have one database why not test out that scenario. If it works at least you have a starting point. Then you can try optimize.
I'm guessing since this is not operational nd you are not using this in production. Why are you using AWS and just out of curiousity.
How many users?
How many concurrent users?
How much storage for moodledata?
What type of EC2 instances / RDS instance are you using?