Seriously, even giving it more thought, I'm not even going to touch Elm and going to keep going with React instead. I don't want to spread myself too thin and value depth over breadth. Just going to run with Golang, React and Elixir/Phoenix and brush up on vanilla Javascript and jQuery with CSS.
I know in interviews they'll ask why I chose to use a certain language for a project, the answer "because I wanted to try something new" won't hold as much weight as giving them the pros and cons of what I chose.
I mainly learn by building projects too. I'll end up picking a language and thinking about what I want to make with it and as build I'll learn how to use the language to add features the features that I want to the project. Right now I'm building a Yelp-type app with Elixir/Phoenix. After that I'll start my job search.
I haven't found too many project-based books, but know they're out there.
Web Development with Go: Building Scalable Web Apps and RESTful Services is a great book that's project-based for anyone else interested in building something with Golang. I highly recommend this one after getting some of the syntax down.
That sounds like a great idea. You are making a great choice in paring down and focusing. When it comes down to it, programming languages are there for you to build something, and it can be easy to lose sight of that and just forever dabble around in different languages. At the end of the day, these different languages just allow us different ways of doing the same thing, which is building software.
I'm going to keep my focus on Javascript/Typescript and C# and really start getting into making stuff and then into subjects like design patterns and data structures and algorithms.