And presumably C# is just binding to some native toolkit

There is no native toolkit on Linux, at least not in the sense that there is on macOS or windows.

Yea but linux doesn't have a toolkit at all, in that sense, and suffers deeply for it. Imho

If I had to simply bind to a native toolkit, I think Lazrus can do that. Having loved Delphi back in the day, I think I'd prefer to write ObjectPascal for anything more than a very simple utility. C# would likely be my second choice.

Yes, but C# is very nice as a language for GUI, can't explain it, just feels right.

Unfortunately, that's a mostly-windows thing. I can't say i've ever used such an experience myself.