A key component in the field of automation was how to ensure a safe and easy transition from our repositories to runtime environments, with as few showstoppers as possible. Azure Pipelines was extremely straight forward in taking a concept from grassroots investigation to production in a very short time, without running the problem of vendor locking, allowing continious use of our current infrastructure, tooling and solutions.
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.
