I love stories that extrapolate on given canon. The art of boxing truly made a man of Dudley and a very good man he has become - which is lovely to see. You make the logical leap of extending Dudley's physicality to his view of life & love and his self-assurance and self-awareness. His dedication to his tasks and his affection for Harry and his family (untarnished by the past) is equally great to see. He learned from his parents' mistakes and missteps.
So much to love here about centaur culture, including their arrogance. You display that well - centaurs have it, wizards have it and even Muggles and each has come by it naturally.
It was lovely to see the centaurs through Dudley's eyes and to find he comported himself better than Harry had ever dreamed. It was interesting how Harry and his kin underestimate Dudley, yet the centaurs took their measure of his character very quickly and then had their hats handed to them at their initially condescending measure of his mettle. That was especially well done. Boxing is an art and Dudley is a master not to be trifled and it's nice to see wizards as well as centaurs impressed by a Muggle discipline.
Dudley's casual acceptance of his sexual proclivities was nice to see, no hang ups or evasions - rather like the centaurs, really. The UST between him and Firenze was quite nice as was the eventual RST. I admit a bit of disappointment in that I misread the warnings and had thought there were male *bj's* in it and had been curious about/interested in your take on that, but on re-reading realised my error. Ah, well!
What was there was lovely and it was especially nice to see they were still enjoying each other's company later in life. Petunia was icing on a very nice cake. I loved her reunion with Harry and their resolving the past and making way to a more fulfilling future. I think she missed kids and might have wanted more of her own if it hadn't been for Harry (an interesting thought in and of itself). Her making room in her life (and heart) for Harry's kids as doting Great Auntie was a sweet touch. It was nice to think she died amongst those she loved and it was very nice to think Dudley was not alone to deal with his grief. Forgiveness clearly has a powerful magic all its own. Well done!