Oracle counts a vCPU the same way on EC2 and on RDS, so the platform choice never saves you licensing money. What it does change is sysdba access, block size and file limits, who owns backups and failover, and the Multi-AZ setting that quietly doubles your BYOL count.
Most AWS cost work goes wrong because it starts with the biggest line item, which is also the riskiest. Order the work by blast radius instead: free networking and storage fixes first, performance envelopes one workload at a time, and commitments last. Includes the commands to find the waste and the cuts that look harmless and are not.
Choosing between Lightsail, EC2 and ECS for a small client workload is not a performance question. It is a question about which constraints you are accepting and how expensive they are to reverse. Profiles of all three with where each wins and loses, the cost levers that actually move the bill, a six-step decision procedure, and the arguments that fall apart on contact with a real project.