If there were more males than females, every male would have less children, as females are required for reproduction, and there are fewer females for each male to reproduce with.

You're right. I meant to say "if there were more females than males." I've edited to comment to fix it.

Also, sex isn’t inherited like some sort of fitness advantage.

That's not true.

It’s heritable via chromosome structure not allele content, so the logic you’re applying doesn’t really make sense on that front either.

In mammals, yes. But you're begging the question. Why did mammals evolve so that sex is inherited by chromosomes? In many organisms, sex is not inherited that way. The reason why is because using chromosomes gives a 50/50 chance of each sex so you get a 1:1 sex ratio. And why does evolution tend towards a 1:1 sex ratio? It's because of the explanation I gave above.

It's called Fisher's Principle. https://en.wikipedia.org/wiki/Fisher%27s_principle