Use a CDN for the HTML and other static assets, that will drastically reduce load on the app server. Flask can scale to 10M users if you use a load balancer, so

Use a CDN for the HTML and other static assets, that will drastically reduce load on the app server. Flask can scale to 10M users if you use a load balancer, so you can build with that. Every cloud provider can scale to 10M users, so your options are open. Read heavy workloads can work well with SQL, consider using read-replicas to scale that if/when you need it. Consider load testing if your worried about scale. You may find lower costs if you use a language other than python or use async python. You may be faster to market if you build with tools you know well. There's not enough info about the project to make recommendations.