Suggestion required for Server deployment

I have built an application on android and will be probably on IOS too. I Am using MySQL, I am a new bee and want your help. The app will work offline but as soon as connected to internet synchronization will take place, the user for the application would be 4k to 6k. Any suggestion for the operating system and hardware specification.

for operating system, I recommend using Linux + Docker containers for both your database (using volumes to persist the data) and your REST/GraphQL API (please tell me the users aren’t directly connecting to the database, that’s a massive security disaster waiting to happen).
This allows you to scale up easily when using Docker in Swarm mode (or Kubernetes, but I personally dislike Kubernetes).
Additionally, it allows you to build automated deployments in your pipeline so that you can more rapidly update your API.
And finally, using containers makes it easier to have a consistent install between your development machine, staging server and production server.