In order to even become a skeptic, you have to know you’re right about being wrong about something.
That requires that you learn you’re right about something in order to learn you were wrong about the first thing.
Therefore it is impossible to be a legit skeptic/agnostic without losing touch with reality completely (at least until you realize you… thinking being… ARE real).
The reason Descartes came up with the ontological argument was because he knew he didn’t think himself into being (he was experiencing a solipsistic nightmare).
It sounds too crazy to say, “I didn’t bootstrap myself into existence, therefore there is an unbootstrapable being.” Because everyone with a bellybutton (or serial number)… needs to know you’re sane before they’ll take you seriously. lol
Not that a solipsistic nightmare is technically insanity… medically speaking. Spiritually, yes. Uncurable, no.
Doubt is not in-itself proof of anything, except in its extreme as Paranoia.
Descartes went as far as that as he could, and if something must be trusted, then it must be the compulsory rational system inherent in The Self, Cogito Ergo Sum.
In other words, it means that the mind/brain/cognition system needs something to latch on, some core-belief, no matter how irrational the core-belief maybe.
This also indicates why so many people are willing to latch onto Mysticism, because they don’t have a superior system of rationalization, or worse, they don’t need one.
People don’t need a superior rationality, when they outsource this to “Experts” and “Authorities”. Most people outsource their beliefs to Theocrats/Priests/Gurus/Etc.
You said he tried to prove something with doubt. No, he didn’t. Doubt rules out. What’s left is not established by doubt.
…”it means that the mind/brain/cognition system needs something to latch on, some core-belief, no matter how irrational the core-belief maybe”
That may or may not be true (you end by saying it ISN’T necessarily lol) but it’s not Descartes’ focus. He wanted to sift away that which is in question and build his worldview from indubitable bedrock.