I think George’s worst decission was not to trust himself enough.
Unlike what most of people think, at times (and only at times) I believe he failed on looking and caring way too much what other people (the industry) was doing when he engaged the prequels. The chosen one thing was taken from Matrix. The Coliseum in AOTC from Gladiator. Qui Gonn came out of someone’s suggestion.
In his initial schemes for TPM back in '97, the Jedi duo was supposed to be ObiWan and a teen Anakin. Had it gone that way without caring about loosing market (sorry, audience) with Matrix or Lord of the Rings, the PT would have turned out a better arc. But from there on, a chain reaction of bad choices took place, of which Jar Jar was just the least important (though perhaps the most evident) of all (if there was no baby Anakin, there was no need of going to Tatooine, or at least not in the same terms where comic releaf was needed to make a light-hearted middle act in a dull desert planet).
There are chosen one types in ancient mythology though, and Edgar Rice Burroughs was having John Carter battle Martians in a gladitorial arena over a century ago. So much has been outright lifted from ERB’s books, that when Disney made John Carter, it seemed like a rehash of those concepts to the uninitiated.
I don’t know about the casting of kid Anakin, since any reasonable director who was listening to the input of writers/fans/other people in general, would have gone with the troubled teenager from the first draft. I suspect that George wanted to be edgy and buck the expectation with his casting of Jake Lloyd as the most Leave It To Beaver innocent youth he could find. I believe his stated reason was that Darth Vader is only a tragic tale if he begins as an inherently good person who is nevertheless corrupted by evil, and if that story were well told, it could have been true. However, child Anakin has little relation to teenage Anakin, who has little relation to Darth Vader. It is a bridge too far, and what really felled Lucas was an inability to recognize and correct his directoral deficiencies.