ujm, this is an opinion. It sounds that ketuba is the right way to do prenup. Ashkenazim our days are using a standard text. It is clear from the gemorah that people used to change value and property that is included, and sefardim still do that, as far as I know.
Avira, I do not have a prenup, so I can’t compare the texts. I can see that there could be prenups that are not in the spirit of ketuba, for example protecting the husband. Although, I would say that conditioning ketuba on resolving divorce fully in Jewish court might bea reasonable idea.
But otherwise, ketuba is preventing divorce exactly by making husband think about financial consequences. Gemora goes thru history of ketuba – first husbands would put money aside in home, etc – but it did not prevent them from making quick decisions.
And you contradict yourself – provision for widows is not simply to make it light in his eyes. Unless you think it will prevent an abused husband from committing suicide as it will not frees him from paying to the hated wife. As in the famous “if you were my wife, I’ll drink it (poisoned tea)”