You've received some wonderful advice about some specific technologies that you should focus on. In particular, the advice to develop strong database skills is

You've received some wonderful advice about some specific technologies that you should focus on. In particular, the advice to develop strong database skills is going to be particularly helpful. I want to add that because you're experience on the front-end, your transition to the back-end will be a little easier. The best way to learn how to design an API is to consume many of them!! Also, since already have front-end experience, you may find it easier to transition from front-end -> full-stack -> back-end only. Your front-end skills will be considered full-stack experience and when you're in a full-stack role, you'll have an opportunity to learn some solid back-end fundamentals! Good luck with this!