I started out with C# and .NET and I loved it. In my opinion, it was the perfect way to start learning the fundamentals of software development however I always felt like I was at a disadvantage when I was doing .NET development. Granted, .NET Core is now open-source and cross-platform, but I moved to Node.js simply because it is incredibly popular. I never thought I'd learn to love JavaScript it the way I did with C#, but I learned to love it pretty quickly, especially once I started using TypeScript. You get all of the benefits of C# and JavaScript all in one. If you've built a REST API with Python/Flask/Django, you'll be able to learn Node.js/Express/TypeScript well enough to migrate your API very fast and it's incredibly easy to host for free on any number of services.

I'm new to Go, I've got very little experience but the 'feel' of Go, isn't like Python in my opinion. Go has a pretty steep learning curve, much steeper than C# in my opinion. So if you are willing to consider Node/Express/TypeScript, I think you may really like it. If you're picking between Go and C#, I'd go for C# as of today, but once I am more comfortable with Go (which I anticipate being a slow process) I may change my mind.

At this moment in time, in late 2020- Node/Express/TypeScript feels like the obvious choice to me as a former C# developer.