I've got a few thoughts on this. Kingdom Hearts 1 was a great start to the series and part of the reason it seemed so grand is because it threw so many concepts at you at once that the universe felt bigger than it really was. More importantly though, the princesses of heart were an integral part of the story. And through the princesses of heart arc of the story, the worlds felt connected because they directly affected each other and the villains in each of these worlds were antagonists of the worlds themselves.
For the rest of the series this is not the case. Instead, worlds exist merely as a setting for the plot to unfold. Can anyone think of a story arc that took place in a disney world, that is important to the main story, that could not effectively be swapped out with any other world and have delivered that story in the same way? I can't. Battles between heartless and the internal and external conflicts of the organization could happen anywhere - unfortunately as the series has progressed this has become increasingly obvious to the point where (I think most people will agree) the disney worlds in Kingdom Hearts 3 felt like a chore - like a mandatory obstacle course to get to the finish line.
I don't entirely disagree with you on the issue of how Disney elements have been integrated throughout the series, and the downward trajectory that seems to follow, but as a matter of general structure there's a reason that undoubtedly flawed games like KH2 and BBS (and their accompanying stories) still resonate in a way that KH3 doesn't: they have strong and well executed hooks bolstered by a clear dramatic mandate.
I think this is best represented by KH2, which features a super ambitious, jam-packed narrative that doesn't always hold up super well to the kind of immediate scrutiny these games naturally elicit as action-adventure epics-- but it's precisely that degree of energy and thematic work happening in the background which makes it rewarding in spite of its errors. This is a game that goes out of its way to place stakes on virtually every event that occurs by tying it into somebody's point of pathos, even creating new characters outright just to keep the wheels on the narrative greased, which enables the action to constantly scale upwards while driving at several core conflicts that, by the end of the game, have culminated towards one final and necessary objective. It's easy to miss this because we're all so preconditioned to recognize the patterns of a story like KH2's and forecast the outcome, but it's worth noting that Sora does not begin the game ever intending to fight Xemnas (like, he doesn't even know who Xemnas is). The game itself doesn't even start with Sora, and so much of his journey is essentially not his own: we view it through his eyes because of the connections he shares with others whose arcs intersect with his, and that robust ensemble element helps the narrative to feel fleshed out with an embarrassment of rich emotion and constant, deliberate payoffs to both its own internal set ups, and those of the past two titles. This broader, galvanizing effort to build the world of KH up into more than one boy's personal heroic escapade helps insulate the lackluster Disney settings from interfering too strongly with the overall momentum carrying the narrative: as
@2 quid is good points out in their essay, the game has a proper, glorious, full on Battle for Helm's Deep style climax at the halfway point and the entire orientation of the narrative shifts as a result. The Organization goes from being a mysterious and troublesome question mark lurking at the fringes of the story's action to stepping up as an imminent threat, pitting Sora against his own best instincts and demanding a refocusing of the protagonists' collective efforts. There's something left to be desired in precisely how this is executed, but the rationale for sending Sora on another round of visits through the Disney environments is solid: by cranking everything up to 11 (or is it 1000?) with the Heartless Battle and then leaning into a kind of falling action mid-narrative, the impression is that the story has reached a dramatic turning point, and even though superficially much remains the same, the underlying tension belies just how firmly things have diverged from where they started. Characters start dying, the worst fears and highest hopes of the protagonists are regularly being referenced as their resolve is tested, and it lands because we have a baseline comprehension of the worlds this conflict inhabits and the effect it's having on them. I think this is thrown into sharpest relief when Xemnas starts calling on his fractured imitation of Kingdom Hearts to give him power, and he's specifically drawing on the lingering anguish of all the people he essentially had slaughtered all throughout the KH universe so that he could collect their spiritual energy and make it his own-- and what's worse, he's been manipulating the good guys into doing the bulk of this work for him. Talk about an antagonist who is
not fucking around.
Meanwhile, in KH3, pretty much all MX does is fuck around. He sends his minions to be pests towards SDG while twiddling his wiggly fingers and waiting for the war to come to him on the barren, forgotten planet where he's ensconced himself at the outer edges of the KH universe, which is pretty symbolic of the tact that KH3 takes towards world building in general. It's all empty throwaway exposition and remote callbacks, or just bizarre plot fudging like the entirety of Toy Box, which doesn't inform or expand so much as it confounds and constricts. There's no turning point because there's no momentum: we go from opening with a fully developed character who elicits an instantly recognizable convergence on themes of nostalgia and loss and the components of personal identity in KH2 to barely even getting an opening in KH3, unless we're counting the filler arc that is Olympus which is so obviously forced into the role of "perfunctory introductory chapter" that Sora immediately forgets what he's doing there from the moment he arrives. And that's, sadly, the theme with the most recurrence in KH3: protagonists set adrift, with no earthly idea where they're supposed to be going or what they're supposed to be doing (and antagonists who aren't much better off chasing not one but two varieties of red herring), and nothing we might learn about the places they visit or the people they see matters because there's no overarching motif to which we can tie it all back. Every inciting incident in the game basically occurs through luck and coincidence (some might even call it contrivance) through totally vacuous dramatic logic, and the Disney worlds are basically littered with stock characters espousing the same platitudes every game has dealt in (and some more embarrassing than those) with none of the context to make it stick. The successful KH games don't work by anticipating a face-value buy-in from the audience on the strength of friendship like some "very special episode" moralizing on the issue: they work by proving the value of that principle through portrayals of how hard-won enduring connections with others come, and how easy (and dangerous) it is to lose sight of them. I guess KH3 kind of forgot about that.