Man, that was a great read, from simple beginnings to selling your product. Thanks for the thorough explanation, I definitely don’t plan to sell anything, I was considering learning electronics as a hobby, but it’s good to know where to start if it ever comes to that.
As a front-end developer with 10 years of experience, I’d suggest going with JavaScript. It’s one language for both stacks, you can learn the core front and back end ideologies, and if you decide to go with a different language for back-end, it shouldn’t take too much time to learn afterwards. From my experience it would be easier both to learn and potentially to get a job in the field.