I don't know. It's different for everyone. I've still never gotten any formal training. I watch a lot of content and think about the game a lot. But never really used a solver seriously or bought a class. And I'm a decent winner up through 2/5. A lot of that might just be 20 years of experience and content consumption.
I feel there is enough free stuff out there to learn to beat these stakes. But it takes some good extrapolation and a inference skills. And it's obviously not all condensed into a single course.
If I had to recommend anyone, I honestly think Hungry Horse has hands down the best content for learning a good process. Bart is good too. And for more online focused, Carrot Corner.
Live NLHE really doesn't require a ton of theory. Taking a course to learn some theory if you don't know the rough basics of it is probably worth the time. But it's really only necessary in live play as a foundation to understanding how and why you want to deviate.
I think anyone that is trying to play online these days is really kinda wasting there time unless you are young and have no job/family. Unless availability is an issue, live is a better ROI than online (except for ClubWPT Gold, which is a honey pot at the moment compared to other online poker).
I think this is one of THE best free training videos I have ever seen...