I need some advice to choose a language for back-end development. Right now, my REST APIs were created by using Flask/Django, and I'd like to create a more reliable and more efficient API with static typing. On the one hand, Go is young, very light, and syntax like Python's, but C# has a large number of libs and more built-in methods. Which is the best solution today?
The best language for you is the one that you know best!
Its a bit of a guess, but from your question and the difficulties you have with Python it seems to me the problem you describe is the manifestation of a bad design/architecture/code quality. These are not the problems of a language itself!
The experience you gained over the past years with your current programming language will outmatch any benefits of another language that you start from zero.
Because in the end of the day languages aren't all that different when it comes to fullfil the same task, it's more the tools, framework and ecosystem for a particular problem that make a difference.
I worked with Java, C#, Go and recently in Phyton, and I would choose Phyton over Go for WebApps, even I like compiled languages more. Go is a very simple language, I would even say maybe too simple. I can't stand all those go boilerplate if err checks, the broken filesystem, the date/time mess and many more things that aren't actually relevant for business application at all. Go has its advantages but not for WebApps.
Keep the lang and improve your skills and architecture you will benifit more from it than from a new language.
From times to times I asked or was asked that same question. Technology aside, it's important to consider the skills and expertise that the dev team has. Whether you use language A,B or C or framework X,Y and Z, if your team has a strong background and experience with something make it count too.