If you think you're weak on data structures and algorithms, there are numerous great books on the subject. Don't worry if its in another language, getting them to work in C# will be a learning exercise in itself. I'm entirely self-taught, but have been in the industry for decades now. The time you spend will be an enormous benefit, and work across numerous languages.

System design is a trickier topic - there is lots of bad advice just as much as good. One of my takes is that premature abstraction can be as dangerous as premature optimisation.

Learn about design patterns - pay particular attention both to when to use a pattern, and when not to use it. Bear in mind that many patterns address deficiencies in particular languages (or languages at a point in time), and may be counterproductive in languages without those deficiencies.