What's the best technical pattern for growing my startup?

I would say a monolithic architecture - the whole backend/api layer in one service. The reason being, for cost efficiency you're gonna pay per server each month. From my perspective early stages of an app don't need to worry about scale, since the primary driver is to acquire more users and a small server pool/ single server should be able to handle initial load of new user/ requests.

The best technical pattern for growing your startup depends on your specific needs and goals. However, one popular pattern is the microservices architecture.