What came before us?

[MiserableSinner]

which tells us the other children were born later.

It doesn't. It says simply that other children were born later. You, however, keep referring to "the" other children, to imply that they were all born afterwards. But at most we can say that children were born afterwards. We cannot say that no others were born beforehand.

Men can have their first child at a wide range of ages, I don't see how a spread of ages in the genealogy contradicts...

As noted, there's nothing to contradict. The spread simply shows that there was plenty of time to have other children beforehand. The text allows it.

Gen 5 is not in isolation. We already know Cain and Abel were born before Seth, so the other sons and daughters Gen 5 mentions who we are told were born after Seth do not include children we have already been told about. They are 'other' than Cain Abel and Seth.

I think it's a mistake to interpret Gen 5 as such, but if you want to do so, then: We already know about the woman Cain married and the people he feared would kill him.