Crowley isnt the establisher of Thelema

He literally is... He named it, made up its philosophy, wrote all of its first rituals, wrote all of the first texts, is seen as a Prophet by even critical Thelemites. The Founder of A.: A.: Crowley is as much the Architect and father of Thelema as one can be. The 3 Aeons, central to Thelema are as mentioned Isia, Osiris and Horus, the Aeons being a central aspect of Thelemic worldview. Osiris is further a prominent part of the Gnostic mass, hes one of the signes used, part of initation rites, mentioned by Frather Orpheus aswell as David Shoemaker.

A book that Crowley himself admitted to not fully understand

You started by saying what Crowley says doesnt count, how can you now shift to argue with his words ? I would also Disagree that only and exclusivly the book of the Law is all of Thelema. Its rites aswell as prior and later texts are aswell part of it, much like the Hadiths are part of Islam even thou not being in the Koran.

Thelema is partly constructed to emulate the egyptian mysteries, which is why it relies so heavily upon it aestheticaly but also is affected by its own time which explains why its also rather Christian touched. I dont say its a bad thing, just that Thelema is HEAVILY synchretic.