The very notion of something being "out there" independent of us is itself a mental model we use to explain our perceptions.

Yes, I think that's right, the conviction that something exists in the world is also a (unconscious) judgement made by the mind that could be mistaken. However, when we what to explain why we have the perceptual data, and it's regularities, it makes sense to attribute it to external causes, but this conviction could perhaps too be mistaken. The underpinnings of rational reasoning seems to bottom out to in unconsciously formed convictions as well, basic arithmetic is obviously true but can I trust these convictions? Justifying logic with logic is indeed circular. At some point we just have to accept them in order to function in the world. The signs that these convictions are ofter useful suggest to me that we have some access to objective reality. But for everything I know, we could be Boltzmann brains floating around in high entropy with false convictions. Despite this, I think the assessment that objective reality exists and that our access and knowledge of it is limited but expandable is a sensible working hypothesis.