Yeah, I think once you have the libraries, you can start to build meaningful projects. I think continuing on to learn some backend will allow you to build even better projects, but at this point you should at least start thinking about things to build - just be careful not to pick something so big and complicated that you get discouraged.
So, yeah, I’d at least want to finish the React section. If after that, you have an idea for a simple React app before you move onto redux, then go for it - just don’t get too distracted on your side quest. And it might be good - it will cement React and also sometimes it’s hard to understand why redux is needed before you build some React apps.
I do like to emphasize that it is important to keep on the path - your goal is to get that foundation in the MERN stack. If you do a sidequest, try to time box it, or just work on it on the side.