I think Rails is the obvious choice for backend apis these days. It's robust, takes care of most of the hard stuff, saves you time and mistakes. Use whatever you want for frontend (React for me) in tandem with it. Many of the job postings I've seen have listed Rails + React.
It used to get accused of scaling poorly, but tell that to Github, Gitlab, and other large-scale sites with millions of users. It's true that bad Ruby is slower than bad Java, but the solution is to write cleaner code.
I know this was all about Rails. I use Ruby for scripting work as well, and it's enjoyable to use.