Internet's full of React courses, but the vast majority of them are aimed to people that have never (or barely) touched it. I already know the fundamentals and built a couple of not too complicated apps to practice with it, but I can't be the only one that now needs more! Anything is appreciated: Youtube tutorials, specific paid courses, interesting reads, etc :-)
After learning the basics, I learned mostly by building my project (social network site) and googling for solutions whenever I encountered a problem outside of my knowledge. Most of the time the answers were in the Stack Overflow, but sometimes I cloned other people's repositories and reverse engineered them until I got my answers. Here's my repository if you want to do the same: github.com/mikey1384/twin-kle Hope it helps.
You need to keep building things either for someone else or for you. More you build more better it will be.
I only started to really feel I was moving to another level with React when I was given a large UI project to tackle and told we had to use React & Redux and bring as much of the code under unit tests as possible. After getting through that and producing something I could feel I truly understood, that's when I felt I could call myself a "React graduate".
Based on this, my advice would be:
Above all, put your learnings to work and build things, solve problems, and have fun! :)
Learn accessibility, learn to do it without the stupid framework, that way you can recognize how much the framework nonsense has saddled you up and taking you for a ride by teaching you bad habits and resulting in websites and interfaces that are a giant middle finger to user with accessibility needs, and how 80%+ of what you've done with the TRAIN WRECK of developer ineptitude has little business being done from JavaScript in the first place!
Seriously, if you think React is something to be used, you likely don't know enough about HTML, CSS, or JavaScript in the first place to be using it.
scotch.io has some more advanced tutorials.
egghead.io Namely, they have a course from the creator of Redux on how to use it.
There are a couple of people on YouTube who do React tutorials and get into more advanced topics like Free Code Camp, or you can glean vague information from developer seminars from AirBNB, Netflix, etc.
You can search for React articles on publications like Medium or Full Stack React. The official React blog is great for keeping up on updates.
A lot of React is all about ECMAScript, so I'd find a few resources to refine your grasp of that.
And as Mikey Lee mentioned, I'd highly recommend searching for React projects on Github and backwards engineering functionality you're interested in. HNPWA is a great site with YCombinator Hacker News clones done in various languages - they have a few React stacks built with GraphQL/Apollo and whatnot. And React.rocks doesn't update much, but they have a few interesting app examples.
Finding case studies from sites that switch over to React is an excellent option as well. Modern Tribe has a decent breakdown of their production of a React/Redux Wordpress theme built on the WP API.