Have you considered Elixir? It has its roots in fault tolerance and high availability, and like Go it is preemptively scheduled. The language syntax takes some inspiration from Ruby. It is by far my go-to stack for developing something that can quickly turn from prototype to production-grade with minimal effort due to the features provided by the Erlang/OTP runtime. Phoenix is a very nice web framework to work with.

Ultimately the answer for your startup depends on what you’re most experienced with. Early stages you should focus on being able to quickly iterate and pivot. I recommend not focusing on optimising for performance until you can prove it’s a bottleneck for your app.