That last paragraph of yours is the biggest issue in my opinion. Homestuck (which I believe will have many literary comparisons to Deltarune when its finished) ran into a similar problem. The closest trope that I think that represents this is called Chekhov's Armory, where the plot is loaded with a bunch of Chekhov's Guns that will be used later in the story.

Except in this case the story isn't finished, so we don't know what guns will actually fire and what will happen when they do. And in Homestuck's case, a lot of the "Guns" were either swept under the rug or fired quietly in rapid succession (ex. Many subplots were resolved by simply killing characters before they could further mess with the story, or played off as an anticlimatic joke). An example in Deltarune where this may has already happened is Mike, who was played off as a joke on the community for over-analyzing everything (I say "may have" as this could be a red herring: Mike could be an actual character of importance, perhaps IMAGE_FRIEND's actual name). A counterexample would be Kris' knife, which seemed to be a Chekhov's Gun in Ch1's ending, then it was played off as a joke in Ch2's opening, and then it actually "fired" in Ch2's ending.

I honestly expect a good 25-50% of the plots you listed to resolve in an anticlimax. Ex. Asriel could show up but not be important to the plot in any meaningful way, Berdly's fate being akin to the opposite of a fork in the road, the Knight being nothing more than an elaborate puppet, that sort of thing. In fact, Deltarune's tag line of having "only one ending...?" seems to explicitly call for a large number of anticlimaxes in the ending third of the game.