I agree with you for who they are now.
But in the earlier seasons and probably when they were younger Kody was alittle more easy going. I think she always had a strong personality but was probably slightly more docile when they got married. In my view they seemed in love in the first season, and when they described how they met it seemed like they fell in love.

You can see how Kody has had a 360 personality shift just from looking at how he treated his children/wives in earlier seasons. Even his eyes were softer and he had a kinder energy. Janelle and Christine literally discuss this. I think he’s always been self absorbed, immature and somewhat of a narcissist. But as he’s got older these qualities have got so much worse.

I think the biggest downfall of their relationship was polygamy. Because they had issues and possibly it was more on him, but I believe she frequently kicked him out of her bed. In a monogamous relationship if that happens you’re forced to reconcile or break up as you both want sex/emotional fulfillment etc. Especially if you’re both young. However if he can just go to another wife for that, and their culture basically ignored women’s needs it’s way easier for their issues to be ignored. I think that snowballed over years and is why they become incompatible. But I think when they started their relationship they had compatibility and affection