This hinges on a presumption that when you have those conversations, your partner is being honest with you. Many of them are not, which is why women often get a shock after they have the first child. Before that the man was doing his share or close to it, but that stops when a child enters the picture because he figures the woman is stuck and can't leave him. He also doesn't like the increased workload a child brings. It's common for men to do an about face once children enter the picture. There can be no warning of this, no red flags. They are excellent liars because they have convinced themselves they believe in equality and sharing domestic duties. But when the rubber hits the road, they really don't, or at a minimum they only do in theory and it isn't important to them. So the choice then becomes leaving when you've just given birth or staying and putting up with it. There is no third option of somehow making him do it. There is nothing you can say which will convince an entitled person to accept a responsibility he doesn't want.
It is indeed about being lucky enough to find a man who is honest about his intentions and has the integrity to do what he promised he would.
So it's not as simple as you make it seem. It sounds like you make a good income, which makes it easier to leave if your partner isn't pulling his weight. Your partner must know that, which could have factored into his decision making. Leaving often isn't a viable option for low wage women with children, so of course they put up with lazy men. It's not about having a low bar, it's about having no good options. Men know that they can leverage your fear of being destitute and your kids suffering to get away with being lazy, even with being abusive.
There are some women who do start with their bar set at sub-basement level, but more often they have to lower it just to keep their heads above water financially. Raising children on one low (or even one average) wage is incredibly difficult and stressful, much more so than staying and just doing the housework yourself.
There is no one size fits all set of rules that prevents being taken advantage of by a partner. It would be foolhardy to think we are immune because we did x, y or z early in the marriage. People tend to be unwilling to acknowledge the huge role luck plays in their success at anything. We want to believe we earned it all. It's a cognitive trick we play on ourselves, because acknowledging we could lose it all because of bad luck is so frightening. However, it is the reality. Your husband could become radicalized by the manosphere, get involved with another woman, start abusing substances, any number of other things which would affect his commitment to his family and to equality within it. Sorry to to say so and I hope nothing like that ever happens to you, but I don't believe it's in anyone's interest to be complacent.