I fully agree with you, I have similar insights (deep technical texbooks are also great), but I have to say that the value you extract is not equal depending on

I fully agree with you, I have similar insights (deep technical texbooks are also great), but I have to say that the value you extract is not equal depending on how you approach it. If what you are learning is purely theoretical and you see zero application, you likely won't be super invested, might skip a few things and will likely forget a lot of them soon. However, if the problem is relevant for you, or even better if you are working on something related, all of the sudden it quickly moves from pure theory to a very applicable thing. I think that's why people who tinkered for years before university if they keep the passion can build insane things during/shortly after the university.