If you're familiar with JS then arguably the easiest transition route for you would be to start building some node based services on the back-end.

It'll allow you to get to grips with some of the gotchas you get with back-end development with an ecosystem you're already familiar with, once you've conquered those you can then start branching out to other languages and frameworks pretty easily with a solid understanding of back-end principles.

Another benefit of this approach is there's a huge demand for JS engineers who are comfortable on the front and back end at the moment, so it sounds like you're in a great position to spread your wings.