RIs like this are great, but the biggest savings we've found is moving everything we can to Spot Instances. We're hoping Aurora on spot becomes a thing as that's really our only remaining RI/on-demand cost.
I've seen some third party services that automate migration to / replacement with spot instances, but haven't used them yet personally.
Going serverless, in many places, has been the most effective cost optimization for me.
Consider a k8s/ECS installation that autoscales.
Each application will have a CPU target. In order to prevent spiky traffic patterns from overwhelming the running containers, we target 70% CPU. As usage goes above 70% CPU, we will launch more containers.
The k8s cluster will have a reservation target. In order to allow fast launching of new containers, we want the cluster to have only an 80% occupancy rate. If more than 80% of the cluster is reserved, we launch more instances and expand the cluster.
So to run my autoscaling container-based applications in a way that will be reactive and respond to incoming load, I have to leave 44% (1 - .7*.8) of my hardware idle. If we also factor in that AWS itself doesn't target a 100% occupancy rate (because then nobody could launch new instances), each unit of CPU I actually use requires a significant amount of idle infrastructure. Easily double, possibly triple in the larger scheme of things. All of that is either directly paid for (k8s nodes) or indirectly paid for (ec2 pricing inevitably has idle capacity costs baked in).
With serverless, we would eliminate many of those inefficiencies. With ultra fast launch times, we don't need to give containers headroom to handle spikes. By not running our compute cluster, we can instantly launch everything we need, eliminating k8s overprovisioning. We're left with AWS idle capacity as the only waste, and AWS mostly has solved that with the spot market.
The math doesn't always come out ahead, but there's a lot of opportunity for serverless to be cheaper in many cases.
How would that work for a database? And have you considered or tried Aurora Serverless?
Serverless couldn't provide enough capacity for us (at peak we use up to 300 vCPUs in this cluster). That was on v1. v2 might change that when it supports postgres support.