I think your points are good ones. I would note though, that:

Even if it's for charity, presumably a professional artist who can command higher fees does better to spend their time on better-paying contracts and donating the money instead of donating their time.

isn't necessarily true when the thing they are donating is their professional work. The reason it's better to hire a good professional is that the professional is going to understand things about design and communication better than the person trying to get the logo designed, including things like how to craft an overarching brand that sends a unified message rather than a single cool looking logo.

I realize it's hard to tell how good a professional is though, and I don't have any advice on that.