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Are you serious here? I just...
An assumption can include doubt. There is literally nothing wrong with that. An assumption does not have to be an ironclad, unassailable, unshakable conviction that something is true. That's literally what a "prior" (as in, prior probability distribution) is: an assumption about the probabilities of things, with the expectation that it not only can, not only will, but SHOULD change unless you got super lucky.
Assumptions are not incompatible with doubt, and I'm deeply confused as to why you would think otherwise. Dictionary.com's usage guide even explicitly says, "Probably the most common meaning of assumption in use today is for indicating a supposition, an estimate, a conjecture—that is, something taken for granted." (Emphasis in original, referencing that "assumption" in English has changed a lot, but always kept a central concept of something being "taken.") I don't know about you, but "conjecture" and "estimate" don't strike me as completely devoid of doubt!