Stephen King is good at exploiting the accidental democratic poetry that arises from common language. A major character in this book is 'The Yellow-Card Man', a name which feels meaningful. A recurring trope is little girls singing skipping songs: 'Charlie Chaplin went to France/ Just to see the ladies dance.' These are turns of phrase which work, or work their way in. He uses this democratic resonance throughout his writing, to give it the feel of being bigger or more permanent than it is, because it is embedded in something larger. In contrast I don't think he needs to insert the tired old 'maggots in the eyes' gruesome shock nonsense to give emotional heft to scenes. It's just not needed, and it makes me feel he doesn't trust his own writing enough. It feels like he's putting it in there because it's an obligation. Having said that - he's one of the most financially successful writers who have ever lived, so I guess he knows what he's doing, and giving the paying customers what they want. There's a little bit of silly gruesome stuff in this novel, easily passed over.
I am not sure human beings, or perhaps modern narrative conventions, can handle time travel stories very well. Most time travel stories have a causal (linear) timeline which drives the plot, even though it is threaded through different 'times'. Knowledge is obtained, items are destroyed, people are saved, because that is how stories work, even though time travel undermines all of those narratives. Not to go through all the ways that time travel stories try to preserve a linear plot in a non-linear world, but they don't often make complete sense. King lurches here between an SF solution (a smattering of physics) and a fantasy/theological solution (to do with destiny and harmony). I find the latter more satisfying in the King universe, because it aligns to the democratic poetry that gives weight to his work. His style of writing naturally suggests there's a pattern to reality, revealed in portentous phrases which catch your ear, and frightful events which recur. So if he builds this into his plot he is working with his style, rather than against it.
And I think this style of writing is better when it suggests rather than resolves. Without giving away the ending, there is an ending. I personally would have liked something less informative, more mysterious and unresolved. But then in a way I am saying I wish Stephen King was less like Stephen King, and he is successful in the form that he expresses himself, so why would he change. I guess my favourite way of resolving this story - this is not what happens - would be if the school teacher came back to the present to find that he had changed the future so much that he was now a novelist called Stephen King, and all his past works had come from the back pocket of the trousers of time.