Dr.GutHealth said:

Fiber trials in this population are thin, and most of what gets quoted is extrapolated from idiopathic constipation in people eating normally.

Dr.GutHealth has the substance of this right. The condition it depends on is worth stating. The mechanism is slowed transit plus reduced intake, and that combination determines which interventions work. Less food means less bulk, slower motility means more water reabsorbed, and the result is a smaller, drier, slower stool. Insoluble fiber adds bulk to a system that is already moving slowly, which is why it frequently makes things worse. Soluble fiber holds water and is the better first choice, and an osmotic agent works with the mechanism rather than against it.

Dr.GutHealth said:

Fiber trials in this population are thin, and most of what gets quoted is extrapolated from idiopathic constipation in people eating normally.

Worth saying plainly that not all of this is benign. Severe constipation on delayed gastric emptying has produced obstruction and ileus cases, rarely but genuinely. Pain with vomiting and no bowel movement is not a fiber conversation.

Taking the question as asked, rather than the general version of it. The escalation order that reflects the mechanism: water and salt first, then soluble fiber, then magnesium or an osmotic agent, then a stimulant only briefly and only if the osmotic has failed. Stimulants first is the commonest error and the one most likely to end in a cycle of dependence.

Dr.NutriCornell said:

The mechanism is slowed transit plus reduced intake, and that combination determines which interventions work.

This matches mine closely enough to be worth saying so out loud. Nothing to add that would improve it.