> Unfortunately, settlements can be much harsher in terms that the legal formulas, and a good opposing lawyer will make them utterly unreviewable by a court, no matter how badly your situation changes, as mine did

Strictly speaking, settlements have to be reviewed and approved by the court, but, yes, they can include terms making the support level fixed even if circumstances change, while most support laws would otherwise allow a party to petition to adjust support based on material change in conditions.