> This is an unpopular opinion, and should be wielded very carefully, but I think something often lost in these conversations is that the existence of a human behavior in an animal is not sufficient evidence that it's backed by the full weight of the human-like mental states.

Here's an even less popular opinion: most human-like mental states are a fiction, so the distinction you're trying to draw probably doesn't really exist. The mental states you attribute to suffering are merely a proxy for the behaviour you see from both humans and paramecia.