A few not particularly relevant concerns. Transfiguration is dangerous in MOR, But the type of danger McGonagall discusses is basically the same as any toxin. It's literally only dangerous if you eat it or breath it. A bubble head charm and enough self control not to eat anything that looks tasty and transfiguration reversion is no danger. So why is Mcgonagall so surprised that Dumbledoor used transfiguration in battle, and is still alive?

That being said, I think transfiguration is second only to time-travel in magics that are dangerous in the hands of creative people. For instance, what about transfiguring antimatter? There's no way Harry didn't think of that. I thought of that when I was near his age and reading canon. Obviously this is incredibly dangerous, but if he wants to level Azkaban, that's the easiest option. There's plenty of other fun things. For instance he could make an ionizing radiation death ray with a chunk of low half life material eg sodium-24 in a lead chamber with a small exit. Set it up as near Malfoy manor as possible and wait a little.