He tried that, a while back. Ontological loops just give him "DO NOT MESS WITH TIME."

Or, if kept at a reasonable scale, fun with pies and bullies?

I'm entirely familiar with the earlier experiments with time and maintain what I said in the grandparent. See the discussion from back then if necessary. The only reasons that Harry doesn't use his time turner carefully, for practical benefit, are narrative convenience and an irrational protagonist. Much the same as in just about any fantasy fiction.

I am overwhelmingly unimpressed with Harry as a rationalist avatar most of the time. To the extent that I'd call "Methods of Rationality" something of a misnomer. He's reasonably clever and flamboyant but his strategic thinking is abysmal.