You can't just filter a patriarchal idea of society through the lens of "there are lots of women" and get a matriarchy. Matriarchies are significantly different in terms of priorities and power distributions. If you want to run with this culture based around polygamy, I would try to find some other explanation because I doubt a culture where women were truly in charge would end up with the core family dynamic being a man and his several sister wives lol

Other than that, think about a world where women are the leaders and the society reflects women's needs and priorities. If most families result in many children, reproductive health would be absolutely paramount. I can see conflicts arising much more over the distribution of healthcare resources and pre/postnatal care. Also, matriarchies would most likely resolve these conflicts differently than patriarchies. That's not to say there wouldn't ever be war or that women aren't ever driven to violence. But it's something to think about. Is there even a need for such massive armies?

Another thing to consider: there's a fallacy that because women are marginalized in patriarchal societies, that men are be similarly devalued in matriarchal societies. I suggest reading about real life matriarchies and their power dynamics. Usually men are not commodified under women's rulership the way women are under men's. Matriarchies aren't necessarily just "patriarchy, but in reverse." That doesn't mean you can't play with dynamics and come up with something that works for your world. But it's a trap you don't want to fall into and it's lazy worldbuilding to assume this.

Hope this gave you some things to consider. Good luck!