It's a stable known stack. It's not hard to find Java developers and the AI Agents are probably pretty good at writing Java too. A Java backend will just sit there and do its job happily forever and you can bolt on whatever front-end you want. Spring Boot has been kind of the standard way to do Java web applications for probably a decade if not longer. It works fine has all the bells and whistles when you're ready for them and most Java developers who work on the web know Spring already.

As for an individual developer doing a side project, you should use Java if you haven't used it before to get exposure to it. It's a fundamental component of enterprise software and if you've never used it before take the time to learn something new.