Open source project idea

Java Spring is definitely an option for back-end but it’s not necessarily required. You could be equally successful using C# .NET Core, Python Django, or something more bleeding edge like golang if you expect to need more concurrency and performance. It is even possible to have Haskell bindings in C/C++ but I suspect the younger generations are more into RUST, Go, and JavaScript/TypeScript mostly. While technically you can also use NodeJS for web API I always recommend against this whenever I get the chance. In fact, I prefer web assembly even for front-end despite knowing frameworks like React, Angular, etc.