My character is too smart, and I hate stories that make the characters do something inconsistent and way below their skill level just so that the story isn't boring. The only real way is to make the personal interactions with the characters more exciting, but my characters are more supplementary rather than the focus. However, my character is highly pragmatic, so he wouldn't do something like one-liners and witty banter and would just kill them before any other action could be taken. Adding a stronger new antagonist wouldn't really make it better.
Potential Girls: Silvia, Hermione, Daphne, Fleur
Chamber of Secrets- destroy diary, diadem, goblet, destroy the locket, have hit wizard use the killing curse on Harry not to harm him and only the shard and get out immediately.
Prisoner of Azkaban- kill Lupin (induced heart failure when transforming by gradual weakening of his heart over the school year) and Sirius(dementor kiss) with events playing as normal except Sirius gets captured, worm tail gets away.
Goblet of fire- destroy the snake and kill Voldemort and Wormtail (all start near the start of the school year undetected with basilisk venom dagger and antidetection magic and cloak of invisibility in the riddle home in Little Hangleton), and destroy the gaunt shack that's nearby getting someone to use fiendfyre on the shack.
He already has complete control over magical Britain financially, legally, and socially. He has a means of combating everyone: the op rings. He has an op skill of portals that would've been obtained before the start of the Goblet of Fire due to his study of a beaux baton for advanced arithmancy and the study of theoretical physics at Pierre and Marie Curie University in France in his second year.
Order of the Phoenix Year diverges from the original story and goes into him fixing all of Britain, and half of the magical world follows suit, but that more is just an epilogue; the only real value of this is to get to see Umbridge put in her place and potentially suffering. I'd also have Dumbledore stupid for his half-Assing of sacrificing the few to save the many when he isn't willing to kill the enemy. By killing the few enemies, he saves the many. Instead, he kills the few on his side and allows the few on the enemy side to kill more of the many.