What would be a good alternative to Go, with

- large and well maintained standard lib - great runtime characteristics - esp. reasonable memory usage - developer ergonomics - matureness - long term stability - preferably managed memory

I really have used a lot of languages. They all have some downsides. I like Kotlin a lot but the JVM is just to cumbersome and resource heavy. Grade is way to complex. Rust is way to cumbersome to write and lacks the standard lib. The only one that comes to my mind is actually .NET Core (C#). But I don't know mature it is on Linux for your standard backend.