If your organization has one API written in Node, another in Java and third in Python without any reason, then yes, all the problems are nails. And sadly, I've seen this a lot.
I was thinking of doing something like that, but how does it work for the company in the end? If they vibe coded their project and now have shitty code full of bugs, you come in, fix the bugs and organize the code better and that's it? How do they continue to maintain it if they didn't have the knowledge to set it up in the first place?
I mean, of course exercise isn't going to fix your vision. But if your vision is going to degrade, you can still choose if you want to live as a fit and healthy person who needs reading glasses, or as a person who has aches all over, is in bad shape, feels tired and like shit all the time, and on top of all that needs reading glasses.
Don't know anything about Scala. Kotlin has null safety and a bit cleaner syntax, but other than that, I don't see too much advantage over Java for backend. In Android, Java is still lagging behind a lot. Also, Jetpack Compose, a declarative UI framework is Kotlin only. Kotlin is also working on wasm (so is Java I think, but Kotlin has working examples with wasm GC) and Jetpack Compose is going multiplatform, including wasm. This video has some examples in description https://youtu.be/oIbX7nrSTPQ
"I have several side projects where I use a simple Spring Boot backend and I feel I can focus more on the fun part (solving problems)."
I tried to do side projects with Spring Boot and I also worked with it professionally. I never got to the point where I can focus on just solving problems, I'm always fighting with the framework, looking for how to do certain things in the depths of blogs and stackoverflow because I can never find what I need in Spring docs. I actually find it interesting how some people seem to be very productive with it, while others have issues similar to mine.