I think Christopher hit the nail on the head when he said, "Grasp what you can and keep moving forward". Realize that you don't need to finish a GT course (or individual lesson) with an A+ score. You can always come back and do all or part of those teachings again later, after you've moved on and made progress in other areas. I've undertaken some of these courses a half dozen times because I'm always learning things that I missed, forgot, or couldn't do well during previous play throughs. And, very importantly, it's really satisfying to discover and experience your own unmistakable improvements. Anders calls this "cycling upward", which I think is a great way to view the guitar learning process.

I realize that this may not answer your question as to when to move on. I too constantly wrestle with that dilemma in my own learning. It's apparently something that every guitarist needs to figure out for themselves. I do know that if you commit yourself to constantly going back and repeating lessons that you're having problems with, the question of when to move on becomes much less important, because you know that you're going to have another crack at everything on your next go-around. Whatever you do, try to avoid sustained frustration. That's a real motivation killer. And if you crush your motivation enough, you're likely to quit playing altogether.

Oh, and as for any of us Boomer beginners playing "as fast and smooth" as Anders or Chris, in this lifetime ..... 🤣

Sascha