You say: "We mature in our ability to imagine possibilities in and outside our own experiences and to critique their likelihood, and we call that "reason"."
It seems from the other things you say that this imagining of possibilities can never lead to knowledge, only to belief.
It seems like you hold to a strong empiricist account of pure reason. I.e. You can only know what you have directly experienced via your senses. Going back to the heart example, it seems like you set a very high bar. You do not know you have an anatomically normal heart because you cannot see it. I'm assuming you still wouldn't know by taking your pulse. I'm not sure on what grounds you could know since an ecg and a CT scan, etc., by which you could see your heart would have to be interpreted to you by experts.
It seems as though you can only hold a very strong belief that you have a heart and that it is anatomically normal.
So, by your method the things we can know to be true are so few that I'm not even sure what they are.
Also, according to your interpretation of how we acquire reason many are excluded from any conversation about what is true. The views of those with a developmental disability or a young child for example are irrelevant here.
I guess I'm advocating for the existence of what we might call 'impure reason' by which we really can know in a meaningful way that certain things are true without directly experiencing them. That doesn't mean just uncritically taking someone's word for it. It means questioning, listening, conversing, seeking, feeling, imagining, creating, living together and arriving together at settled judgements about the truth of things. (Your model seems very individualistic to me btw). Those settled judgments don't enable us to say that nothing new will ever be found but they do enable us to say that the new thing to be found will not be, for example, in the case of the Holocaust that the event didn't happen at all. It will be a new understanding of or a new facet or meaning of an event we know did happen.