I have worked in a Spring environment for many years and I still love working with it. Super quick to get a base application running and get coding, thanks to Spring Boot's easy and straight forward integration with Tomcat. However, I will try and answer this question from another perspective: look at topics such as popularity of the language, average statistics on community contribution to their repositories and hiring availability from companies. If you are going down the API route for backend, leverege on your experience in the Node world by looking into ExpressJS (or even NestJS). The JavaScript world is really excellerating at a lightning speed, and I could recommend exploring those worlds a bit more, should it be a comfort level for you. However, my biased answer is tryout Java, followed by Spring afterwards. No disrespect to any .NET developers out there, as there are a few topics in there which are fantastically implemented.

I'd recommend to learn Spring as it is very widespread in the industry and provides a lot of easy integration into most of the common backend tech stacks. Rather than learning Java you could look into Kotlin. It's a very consistent, stable and well-thought language in my opinion and not as verbose as Java. Many problems can be solved with Kotlin in a clear and elegant way while also always having the option to use data structures and libs in JVM. It is also has a very good support in Spring.