Excellent plot, but too brief
I think this story is an example of the importance of "show, don't tell". Because while the idea behind the story is exquisite, without any obvious plot holes and no technical faults I could pick up on - it reads very flatly because it's essentially Harry giving a summary of the events without giving much opportunity for the reader to see the events as they happen. At the very least there should have been a little more show at the end when Dumbledore showed up to face Harry a last time - there's really no excuse for just glancing over that part when the narrator himself was there in the middle of the action.
Read the story now
Other ways to divert from the telling would have been to, for example, reference how the golden trio in the original books would often give each other exposition when necessary. Start off with Harry doing a minor exposition dump on the backstory to Hermione and Ron and then cut to a flashback of when Dumbledore shows up. End of flashback and Harry wraps up any loose ends with more exposition if necessary.
But like I said initially, the core of the story is really great, and it certainly made me want to read a full fleshed out version where we get to see the scenes of Dumbledore interacting with Grindlewald, how he faked his own death and how the showdown between him and Harry went. If you ever decide to flesh out the story and show all these scenes you've told us about then I would feel most privileged to read it, because I think could be a true gem.