We considered Go! At the time it was much more designed for servers than mobile devices. If I recall correctly the minimum binary size was like 30mb or something.

I think you are missing the fact that Dart is actually an incredibly nice language to work with in a way that Go absolutely is not.

The problem is that it’s yet another language. It’s the cognitive load and the inability to easily reuse code across a project.

That seems more like an argument against Go. Dart is the language more familiar to the average dev, and you're gonna have an easier time translating that to Java/C# than you are Go.