I've also heard for side projects when you want to learn a new technology to experiment with one piece of it, so you're not totally overwhelmed, and so you can

I've also heard for side projects when you want to learn a new technology to experiment with one piece of it, so you're not totally overwhelmed, and so you can see how it works with stuff you know. For instance, use the rails backend deployed to AWS that you know, but swap out angular for react. I think this works for your question pretty well - if the side project becomes a money-making project, at least there's only one part of it that will be a technical risk, and often if you need to do something quickly, you can work around and do it in the parts that you know.