How to estimate AWS billing based on monthly active users?

Hello.

You should be able to use the AWS Pricing Calculator to estimate the cost of AWS services.
You can calculate the price by searching for AWS services with this tool and entering the information required for calculation (specs, user access, expected requests, etc.) for each service.
https://calculator.aws/#/

Additionally, what tools or methods can I use to test if the platform can handle up to 18,000 users simultaneously?

For system load testing, I think you can use Distributed Load Testing or Apache JMeter.
https://aws.amazon.com/solutions/implementations/distributed-load-testing-on-aws/?nc1=h_ls