What can I do if I’m concerned that my ex, though well-intentioned, won’t follow through?

Kids are vulnerable, even those without high needs. If you don’t stick up for them and raise all relevant concerns in your agreement, who else will? Your first job is to find a facilitator who will ask literally hundreds of questions to help you think of almost everything. Once all those details are in a binding contract, no one can weasel out no matter what.