What Kant is saying is basically that categories like cause, and concepts like substance, are fundamentally bound up with the structure of our mind. We cannot "get outside" our own minds to know anything else, but there must be something beyond our perceptions, which Kant variously calls noumena, things-in-themselves, or transcendental object=X (there are minor differences between these notions that I cannot recall). Here IIRC his language kind of unfortunately but necessarily breaks down into metaphor. He can't quite say that the noumena are the source of phenomena since that would imply a causal relationship, and he's just argued that cause is a structure of the human mind.
Considering the "perspectives" you mention, e.g. that of the flat earther - a perspective is not a perception, nor is it a judgement, and it is these that Kant is primarily concerned with in the 1st critique. I'd say a perspective is more like an agglomeration of more or less (but not necessarily) coherent and self-reinforcing beliefs, attitudes and probably even non-propositional content like dispositions. Kant didn't have much to say specifically about that in any of his major writings that I can recall (maybe in the lectures on anthropology).
But his entire life work was devoted to how we can provide a solid foundation for claims of knowledge, morality, and aesthetics, so to say that "Nobody, ultimately, has any right to pronounce objective truths on anything" would quite a misinterpretation. You could argue "well but it isn't objective knowledge since the noumena are the real objects" but that would be begging the question against him, his whole argument is that concepts like "object" are part of the structure of our mind. Objective knowledge and the human mind go hand in hand.