H.,For your usecase a push mechanism would be better than a API based communication. You can leverage Lambdas to push data periodically. Pushing updated once every 30s would mean you would be paying significantly less on compute costs ($0 on free tier, <5$ thereafter). This also reduces load on your existing DB - and you dont need to manage complex indexes for changed done in the past X mins. Depending on the throughput and technology you are using for the application that writes to Oracle, you can either query the DB directly, or use event sourcing or CQRS to ensure data consistancy.
PS I am new to freelancer, hence no reviews.