C# is what I use at my day job and I think it's a very good language. It's feature rich (some people make fun of it for the fact the language team keeps adding new features). If you are familiar with Python and Typescript, that means you are reasonably familiar with object-oriented design. So, depending how you look at it, it may be very rewarding (since you will be able to quickly build on things you already know) or boring (since conceptually you will be mostly doing OOD). If you are interested in this path, Kotlin may be another interesting language which looks quite similar to C#.
Functional programming is all the rage now. So, if you want a real challenge that will help your programming skills no matter what, you may want to start looking into a language in this family. Haskell and F# come to mind. But there are many others.
Rust is also very trendy. In some instances it's replacing C and C++. It makes one think about lower level details of software. It's also a good language to learn if you want to be familiar with WASM. So, it may be an interesting challenge to take. Lemmy is written in Rust, so you may as well contribute to the platform. There is a lot to be done :)
If you don't already feel proficient in SQL, that would be an important skill to obtain along the way. Databases aren't going anywhere and it's a whole different world.
That said, you may want to focus on general things like design patterns and development methodologies (TDD, etc.) to broaden your horizons. Writing code is one thing. Writing high quality code with few bugs is a difficult thing to do and a great skill that only comes when one understands the best practices of software engineering.
Hopefully this helps. Good luck with your endeavors.