PI coauthorship on previous papers
We cannot tell you what the best decision is, but I get the feeling from your post that you already have some course of action in mind: politely decline, with your arguments, and if he insists too much, find another advisor. I would agree with that being the best solution for you, but only assuming that your assessment of the situation is correct.

Don't take it badly, but I have seen it happen before. An established researcher gives advice to a newcomer (or more junior researcher), who disagrees and concludes that such bad advice can only be motivated by self interest. That sure is one possible explanation, but another might be that the advice is (partly or fully) correct, but the student doesn't recognize it (yet). Take a second look at his advice... There can be good reasons to suggest nonstandard notation: trying to make the paper more appealing to the community of another field, by using their notations. Or making the case that existing notation doesn't properly cover the cases considered. Or maybe what you consider standard notation actually isn't, and you are currently using a notation used by a prolific research group (bias in your reading list).

I am not saying that it is your case. I'm only saying that you should seriously consider it before taking any action.