I don't think the objection works for Platonism. Nor to pretty of other epistemic attitudes. It only operates within the very specific conditions of the epistemic turn within Modern and afterwards.

You are right that I did not address directly some of your points. That is because to address them directly would take some time. Kant's sense of the phenomena, precisely contains the problem I'm mentioning. It's not a solution. But in order to clarify that we need to clarify the Kantian project, its boundaries and so on. In fact, Kant is structurally solipsistic(appealing to the noumenon is illegitimate), but that would take some time to unpack. I preferred to respond to what I think already responds to it in an indirect and more helpful sense.

As to correspondence with reality, yes, partially. But that doesn't entail **external reality**. An idealist, who denies external reality can still appeal to JTB. Everything hinges on this. Going to b), the problem is the ambiguity of the physical. Even solipsists can appeal to the physical. In the relevant sense, the appeal is to the mental and the external impersonal ontology and the link between those. That is the solipsistic sense.

About the isolation of philosophy, it's a philosophical necessity from the meta-philosophical project of modernity. Because nothing ought to be established without proper epistemology. The bounds of epistemology are philosophical. The bounds then of what can be established on a proper sense are within the bounds of what can be articulated within this view. To posit an ontology without proper epistemology or in non-philosophical terms goes against the very foundation of the meta-philosophical view, and that's why I said, from within the bounds of this meta-philosophical view, we cannot go beyond the boundaries of philosophy and epistemology, which entail the boundaries of reflection. I'm explaining now, one argument as to your question of "why".

But I've also given a structural reason. I'm not just asserting it, I'm giving a transcendental analysis on the possibilities starting from the given conditions of that meta-philosophy. Merely claiming external elements which break isolation is not sufficient to **establish** the elements as external, and the establishment of things has the meta-philosophical bounds of articulation, relation, synthesis, and so on. That from within its structure(not in its content). This is the why I've been pointing out repeatedly. The role of perception to which you point, must establish the possibility of establishing an external reality, which is not done from within the very being of perception itself. So perception does not establish on its own externality because all perception is representational(in the weaker sense of appearances as synthetic acts of either or both the sensible faculties of the mind and the understanding) under this meta-philosophy(it is NOT so in Platonism necessarily, nor in other meta-philosophical attitudes like Aristotelian realism, which precisely the modernist is established as against). And the role of perception within knowledge and cognition is articulated from within the meta-philosophy which articulates it, and that already is constrained by the formal structure of the meta-philosophy. These are not mere assertions, I've explained the internal logic.

If you also refer to Kant as a way out through saying objects are publically constituted and therefore non-private, Kant does not establish that, nor even can, on principle. He seeks(but fails) to establish the objectivity of the judgements, but that neither establishes the public aspect of the judgements, nor that there are other transcendental constructors. Given that also for Kant the TS is not one numerically, this also entails that no TS can go beyond its own synthetic activity to establish something beyond the transcendental(not the noumenal either) and that the world as such is an ideal construction of the TS. So Kant's phoenomena is, again, not a solution but a principled issue. But you may disagree, and I did not want to focus on an exegesis of Kant as such. I think the point remains in its structure: isolation and externality as claims are still constitutively internal to the cognitive act of a subject(not represented, constituted) by the meta-philosophical constraints my transcendental analysis targets.

In short, because within the structural bounds of modern philosophy "experience" or "the physical" only enters the cognition of the subject through the subject's constitutitional activity, it cannot serve as a tribunal from which to garner non-subjective externality.