Hiya! OP is spot on about historical info, mythological depictions, and practitioner records being only a piece of the puzzle.
“Preparatory immersion” (which spans history, mythology, and the magical journals of others) has no other purpose than to link you to the energetic conditions which allowed the discoverer to perceive things in the way that is written down. Even if you don’t believe what you read, the act of research creates an unconscious expectation which resonates with the description; you vibe with the author or account in order to access what they got.
That isn’t reality, though, that’s just piggybacking! The people who get into magic to accomplish normal things more easily usually don’t need to go deeper than other people’s work, because if what they access manifests what they want, they either have no impetus to take it farther, or continue going by what worked for them to get what they want.
When you peel back the layers, you find that there’s a lot less individuality and distinction than mythology and history would have you believe. There is no such thing as a ‘line’, much less one to draw- everything available for magical reference is just what the authors could access and understand. Our complete recorded interactions with spiritual forces are really just a pile of boxes we stack together and draw lines between, because of our obsession with categorization and control- there are no lines, much less the lines we use to make the boxes we stack!
‘UPG’ is usually relegated to shamanistic or mystic fields of esoteric interpretation, but that’s just another box. What it really means is to believe in yourself- don’t trust ‘the powers that be’, trust ‘the powers of ME’!
It takes some getting used to, but when you’re used to using yourself, there’s no other feeling like it. You’ll stop treating people and their records as gospels and guides, and starting using them as they were meant to be- the observational equivalent of rumors! Some are fruitful to follow and flesh out your work, but more of them are just ‘loops of interest’, energies that are pretty and flashy and mildly interesting but otherwise useless.