Hello Ben, all,
We live in 21st century. There are many good Dhamma books and suttas available some of which are for free. Why can't a person read a lot of good books and practice based on them? I understand that traditionally, centuries ago, there were NO books and no internet available so the only way one could learn was from a Teacher or a monk. Today the situation is different. It is possible to know more than a teacher by having read Nikayas, Abhidhamma, Visudhimagga, and many good meditation books. Regarding "personalized instructions". Typically on the interviews in popular methods the suggestions were all the same "just observe!" or "don't forget to label that". It seems rare to find someone who has different suggestions for different occasions and who can tweak instructions specifically for you rather than repeat the standard teaching in that Center. These are not hidden instructions and one can learn them, and more from books.
I also have a hunch that progress in meditation does NOT equal knowledge of more theory. So the meditation master doesn't have to know more than a newbie, the difference is in the skill at applying the basics. It is like in a case of a boxer. A beginner boxer who later becomes an advanced boxer can know exactly the same moves as before, but the advance boxer is better because of having practiced those moves hundreds of thousand times more than a beginner and can use them better.
With best wishes,
"Life is a struggle. Life will throw curveballs at you, it will humble you, it will attempt to break you down. And just when you think things are starting to look up, life will smack you back down with ruthless indifference..."