Seems like you know the answer. It probably depends on what your goals are…
If you intend to make money and or invite a team into the project over time then maybe look for something lots of people use (.Net Core, C++, TypeScript, Java, etc.)
If you want this project to be a labor of love that is mostly powered by you then pick whatever esoteric thing you want that fits the bill (again sounds like you know the options, Rust, Crystal, Nim, Server Side Swift, etc.)
If you intend to make money and or invite a team into the project over time then maybe look for something lots of people use (.Net Core, C++, TypeScript, Java, etc.)
If you want this project to be a labor of love that is mostly powered by you then pick whatever esoteric thing you want that fits the bill (again sounds like you know the options, Rust, Crystal, Nim, Server Side Swift, etc.)