Since you're a frontend dev, you have experience working with async processes. ASP.NET Core is reactive (async) by default, giving you a higher throughput out of the box. You can do the same with Spring Boot Reactive, but it's less common. Typescript knowledge will get you comfortable with any OOP language, so both C# and Java should be fairly easy to work with. Then you should research what you would like to do with backend skills : do you want to work at huge corporate (Java) vs government (C#) vs startups (C# or Node.js) ? Do you want to work as an employee or a freelancer ? Check out what are the job opportunities in your area. Then you can choose what's best for your future goals.