Critique of Pure Reason is exactly about metaphysics albeit as a critique. Modern philosophy at least in the transcendental aesthetic doesn't supplant metaphysics.
> 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.
It’s trivial to criticize anything you’re not familiar with but that’s certainly not the way good philosophy is done.
Your hypothesis on knowing presumes that we have knowledge at all and that rationality leads to truth. Secondly, it’s nebulous because “to know” is, given the parsimony of the statement. Your argument only seems logical by the ambiguity of those concepts, “know” and “knowledge”.
Thirdly, these knowing agents are often victims to fallacies, delusions, and severe hallucinations—this strong naive realism has issues noted by even the early Greeks.