Ask HN: What to learn after HTML, CSS and JavaScript?

I am a backend python developer who recently picked up some HTML, CSS and Javascript skills with intention of building my own web application/ saas product. I am confident with my basic front-end skills now(build a couple of static websites). Should I jump into learning reactJs now ? Is react an overkill for building medium complexity web app ? If so what other frameworks should I consider learning? Any suggestions for learning resources for the same?