It is most certainly not circular. We are knowing agents first and know things spontaneously. Only on reflection do we begin to ask what knowledge is, what the limits of rational inquiry are, etc. But to be able to reflect on something, there must first be something to reflect on. Thus, we must know something before we can begin to consider epistemological questions or even have those questions in the first place. Methodic doubt always leads to incoherent conclusions because it is ultimately irrational.
> Not at all, I suggest to read Critique of Pure Reason.
I am not surprised by this recommendation given what you said. The Kantian position is extremely problematic and your appeal to it explains your reaction.
> I'll also note that epistemology was well established by Western philosophy long before Descartes.
This was never my claim. I only claimed that epistemology is not an ideal starting point. Obviously, epistemology predates Descartes. Modern philosophy onward (that is to say, beginning with Descartes, traditionally, anyway) shifts its concerns away from metaphysics to epistemology. Thus, epistemology supplants metaphysics as what one might call the primary concern of philosophical inquiry.