@ say:
You bring up some excellent points. It's true that Descartes didn't quite take Self-enquiry to its conclusion. And it is also true that there is no such thing as "Higher Self". While it is somewhat correct that we are Existence itself, the real Self is beyond even all forms of creation (the body, the mind, the universe).
To use the terminology of Vedanta, enlightenment is when the Self comes to recognize itself in the context of an individual's life. That moment of recognition severs the
ahamkara, or the exclusive attachment to individual identity.
Ahamkara is not the psychological ego; it's merely the attachment to identity, not the identity itself. Ahamkara is the false notion that "I am this individual who does things".
Once that bond is broken, you recognize that you are the Self (that which is the formless, attributeless entity that pervades all phenomenal existence). But you remain the person you've always been, minus the exclusive attachment to the idea of "I" as the basis of identity.
I highly recommend you read the book "I am That", which is a compilation of talks with a Realized Master from India. The lucidity of the answers is unparalleled. You can also
read it online.