I still work on a lot of Java projects. As of JDK 17 Java has most of "ML the good parts" and has the same scalable, reliable and high-performance threading Java is famous for. JAX-RS provides a Sinatra style framework that makes it easy to write JSON API back ends. JDK 21 is just about to come out as a long term supported version and it will be even better. .NET is similar in capabilities, I think C# is a better language but Java has a better open source ecosystem.

I do my side projects in Python with aiohttp and think it is a lot of fun even though people tell me it is suicide (I guess if you block the thread you are in trouble)

I think "Next.js" really wants a node.js backend which has the big advantage that you can share code with the front end and back end. It's basically single-threaded but I know people who are happy with it.

The system I'd most like to try is

https://www.phoenixframework.org/

which is just great if you want to do stuff with websockets that is more interactive than what most people are doing.

I think though that developing that many different front ends would drive you crazy: is there a real reason you can't just make it a web app?