I don’t consider myself a feminist any longer for many of the reasons that you cited as well as some others, although I still support equality for all and the freedom for everyone to be who and what they actually are without artificial barriers to that.
Your ideas about poly are off-base, however. You are trying to apply a polygyny model and it doesn’t track. Because both people in any relationship have a theoretically unlimited number of partners — not just the man, you would not end up with one man with 40 wives, because the wives would all have other partners too. I have a solo poly friend who has three main boyfriends, although she has sometime connections with other men and women also. Each of her boyfriends also has 2–3 other partners. One has both a boyfriend and a girlfriend. Sexual fluidity is much more common in the poly world than the vanilla world. I have another friend who lives with his girlfriend and her husband. I have two male life partners, and a girlfriend that I’m in a throuple with my husband, as well as sometime connection with other people. It never would go in a polygyny direction if it were normalized because it’s a totally different paradigm. It’s non-hierarchical. Each individual is co-creating different types of relationships that work for the people involved in them, and there are all different kinds of relationships that meet different types of needs. For example, I have a really different relationship with the partner that I live with and share everyday life than I do the partner who now lives on the other side of the country from us. They are apples and oranges, but they are both still my committed life partners.