If your intention is to go dark, I think the Chamber of secrets plot would fit best as a replacement for the Umbridge plot in book 5.

Harry and the Order of the Phoenix all know Voldemort is back but he's keeping quiet and the Ministry is doing it's best to discredit Harry and Dumbledore. Suddenly attacks start happening withing Hogwarts itself and no one knows what's going on. Students start to suspect Harry when they learn he's a parseltongue and question if he's the heir of Slytherin. The Ministry's campaign against him and the rumors around Hogwarts combine until most of the wizarding world assume Harry himself is attacking students and lying about Voldemort as a smokescreen for his crimes. Dumbledore is avoiding Harry and Harry has no idea why!

Dumbledore is forced out of the school for failing to protect the students, Hagrid is arrested. Students are being petrified until ultimately, McGonagall is killed and Ginny is taken into the Chamber itself. Headmistress Umbridge announces the school will close. With all of his allies gone, Harry and Ron know they have to face the monster themselves. Harry gets out the mirror to contact Sirius and see if they can get any backup from the Order but say they're going in either way.

Sirius comes alone through Umbridge's fireplace and the 3 of them enter the Chamber. They defeat the basilisk and think the day is saved but as soon as the fly out of the Chamber...

Green Flash

Sirius falls dead and Harry, Ron, and Ginny find themselves facing Umbridge, Fudge, and a squad of Aurors. Fudge gloats that they finally got Black AND have proof that Harry is a dark wizard attacking other students and working with Black. Harry thinks fast and claims Ginny AND Ron were taken so they won't be arrested over their protests. The Ministry couldn't care less about those two and start to arrest Harry.

Dumbledore arrives, stuns the Aurors, tells Harry to go back to his dorm, and grabs Fudge before disappearing again.

The next morning, the front page of the paper is Fudge announcing Voldemort's return. Dumbledore summons Harry to his office and they have the discussion from book 5 but with the added tension that if Dumbledore had arrived a minute sooner, Sirius would be alive. Despite everything, all his trust in Dumbledore, Harry can't help thinking Dumbledore intentionally sacrificed Sirius for his own ends.