Of the two options you mentioned, only Django is a backend technology. AngularJS is another toolkit to build web applications, though more of a framework with a complete set of technology than Bootstrap which is just a UI library.

I think that the best way to push yourself forward is indeed to learn multiple technologies and multiple programming languages, and Python is often a good choice - it is simple, clean and has a lot of mindshare (i.e. you can easily find help).

So when looking at the next technology to learn and the next programming language, Django is a good choice. If you want to stick with Javascript, then I can echo the recommendation to look at Node.JS as a backend service - though if you want to push yourself and become a better software engineer, then putting all your eggs in the Javascript corner is probably not the best choice. Good engineers have a large set of tools to draw from and knowing more than 1 language can both increase your marketability and tech you to think in more than 1 way.