I'd recommend continuing to look into .NET Core. It's really quite stable these days and has all of the features you've mentioned, as well as C# having a lengthy history as a language. Knowledge is relatively transferrable amongst JS (especially TypeScript) developers, as well as Java developers. This puts you into an area where finding capable people is easy and cost-conscious (vs. niche languages.) Historical baggage has also been trimmed with .NET Core, and Microsoft has done well maintaining the language over time (adding new concepts and removing old ones.)

In a couple years, .NET Core will be even more stable and will surely have convinced the stragglers in the community to adopt it. (I'm seeing this is already mostly the case, because everyone likes not being locked into Windows.)

I'm a pretty big fan of .NET Core (coming from Node.JS) and I really do expect it to grow a lot in the next few years as people discover how easy and sane it is.

C# has always had "compilation to a dynamic native binary" since version 1.0 via NGEN.

Mono has always supported "compilation to a (possibly static) native binary".

Windows 8 introduced "compilation to a dynamic native binary" on the Windows Store via the Bartok compiler used in Singularity.

Windows 10 adopted "compilation to a static native binary" with .NET Native, based on Midori compiler toolchain.

The .NET Core team has CoreRT for the same purpose.

> it's definitely a "Windows First" language

C# used to be that but I don't think that's the direction anymore.

Which platform-agnostic GUI toolkit do you use?

HTML but there's always GTKSharp: http://www.mono-project.com/docs/gui/gtksharp/

Which platform-agnostic GUI toolkit do you use for Go? What about Python?

No language ships a cross-platform GUI toolkit in its standard library.

TCL comes with TK, Python comes with PyTk, Java comes with JavaFX...