I'd say give one of the broad categories a try, instead of languages/tech stacks. I don't know if you're employed and if you can either switch companies for this sake/choose within your current company or you do independent consulting. But whatever it may be, try web apps for a year you'd automatically learn JavaScript, HTML, CSS and some good enough framework, maybe try mobile dev for year along the way learning Swift/Java for Android or try enterprise projects in Java and learn a bit of spring/akka or something like that.

My story started a little similar to yours, I was a C++ developer working on modems and drivers, then worked on AngularJS in 2014, then rode the initial Big Data wave and learnt Hadoop, Apache Spark and a little bit of Scala (very scared even now), then went full on enterprise Java building batches, services, message daemons etc. for quite a long time. And now for a year and a half back to JavaScript after half a decade on a mammoth React based project, very surprising to see how much JS ecosystem has evolved. I enjoyed developing pretty much all the projects I worked on, learnt quite a bit of Java, JavaScript, small amounts of Python and Scala.

On the question "How to decide?", I went by 1. hype, 2. interesting and colourful stacks, 3. make me good money. Went just head straight into one of these based on what I felt that year/period