Looking for a best-practice for building serverless application with amplify and api gateway and lambda

A customer is building a trivia app for sporting events. One component of the system is a mobile app that communicates with a back-end to get the questions.

They are building the app using Amplify. This will communicate with API Gateway which will in turn call Lambda functions. They are aware that people may try to break the game and are looking for some security best-practices for building the app.

So far, we have been talking about

• Use WAF with API Gateway
• Store secrets (such as DB connection strings) using Secrets Manager
• use Cognito or similar authentication to secure connections with API Gateway

Are there other services or approaches that people would recommend?