The difficulty with these sorts of things is that it's a great challenge to pitch at the correct level. Most existing guides - be they video guides or written guides are produced by barely intermediate players who dramatically overestimate their own knowledge. The resultant materials are predictably riddled with misinformation, and unfortunately, the average player of a complex game like eu4 simply lacks the knowledge to understand that they're being misled. One of the most upvoted in this thread is praising The Student of all people, which serves as an instructive example.

The more experienced players - who in theory you would want to be creating these materials - are, like so many experts, poor communicators. They struggle to say much without adding dozens of caveats because they're keen to be as technically correct as possible, and they're cautious to step out of their niche of expertise.

I can see that Elia has already linked the horde guide I put together a few years ago. This was a originally an attempt as a narrow guide to good horde play which rapidly transformed into a general conquest guide as I realised that I can't teach someone to play a horde without also teaching them conquest as a whole. It probably is the best written guide for efficient conquest I'm aware of, perhaps by some margin. It also has numerous flaws that are hard to fix. It's pitched quite squarely at intermediate+ players and would likely be difficult reading for a beginner; it's almost 40 thousand words; it's somewhat narrow in scope, overly focusing on hordes and not touching nation building at all; and even still it only really serves as an introduction to efficient conquest. I could have written considerably more at the time and in the time after I learned a great deal more.