I am not saying it did not sell. That's the one thing it did really well. But it's hardly a hot take to say success is not a measure of quality. Plenty of mainstream slop out there. HP is slop. It's not offensively bad, but it's certainly not good.

Over 6+ books it's really sub-par writing to have a character who does not really grow because they already did not have any internal flaws or conflicts. The upside is that it's really hard to hate a blank slate MC and you don't risk writing yourself into a corner. I'm sure this is no small part of why there is so much HP fanfic specifically -- it's hard to write those characters badly as they lack so much depth!

Tons of things did the HP formula better, with well developed characters, good worldbuilding, good plot, good themes, yada yada. e.g. The Magicians (only saw the show) or Misfits&Magic. And in all of those the protags face strong personal hardships and are drastically different people by the end. Yeah, it's hard, but that's what storytellers do.

I’d argue Harry is way worse than both Aragorn and Rand Al’Thor.

That's fine. You're entitled to your own opinion.

What qualities does Harry have?

Naivete, isolation, and confusion that gives way to optimism and comradrie in Book 1. If you ever read any Roald Dahl novels, he's got much of the same youthful curiosity and compassionate cheerfulness of James from the Giant Peach and Charlie from the Chocolate Factory.

Much of Harry's early personality is informed by his struggle to understand his parents and his parents' friends, picking up and discarding their habits and traits in pursuit of self-actualization (Book 3/4/5, in particular, have him latching onto Remus Lupin and then Sirius Black as idols, only to lose them and himself in turn). Over the course of the series Harry's initial optimism is poisoned by cynicism and hatred, frustration at the failure of his elder peers, and ultimately a depressive death spiral. He matures, discarding the childish qualities of the early books and adopts more mature (often toxic and reactionary) views and motivations by the end of the series. As a case in point, Book 1 Harry would have happily joined SPEW, while Book 5 Harry considers it an annoyance. I'd say Harry's arc really peaks in Book 6, when he uses black magic on Draco Malfoy and Snape has to rush in to save him. He's gone from a cheerful, generous, naive little kid to a battle-hardened child soldier.

Like, if I was to really describe Harry's story progression, that's it. Its a look inside a child that's forced to fight a war for survival. You get a similar (abet much better written) character trajectory for the Animorphs. But to say nothing is going on with the central character? That's blatantly rage-bait.

Also my suspicion that book 6 is the last book that Rowling had more than a few token notes on. By book 7, you can really feel the ghostwriters crowding in and WB taking a heavy hand in editing/finalizing (although it's clear they've been around since book 4). Forcing a Disney-style happy ending on a wizard civil war betrayed so much of what Rowling had set up in the early novels.

Literally "the pre-destined future leader who just needs to walk forward and automatically wins".

What's his biggest hurdle through the entire three-book adventure? Picking which hot princess he's going to marry? Politely asking some ghosts to defeat half the Dark Lord's army in an unwritten side adventure? Literally walking up to the Black Gates of Mordor and telling the Eye of Sauron "Made you look"?

Come on. The most difficult fight Aragorn has in the entire epic adventure happens in the first half of the first book.