Dr.NutriCornell said:

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

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.

mia_MS2 said:

Constipation replaced nausea at around week five and has been the more persistent of the two by a wide margin.

Acid reflux and constipation: I developed GERD symptoms around week 7 that I'd never had before. The delayed gastric emptying means food and acid sit in your stomach longer.

Management that works for me: elevate head of bed 6 inches, don't eat within 3 hours of lying down, avoid trigger foods (tomato, citrus, spicy, chocolate), and famotidine 20mg at bedtime. Much better now.

VendorMark said:

Worth saying plainly that not all of this is benign.

Gastroparesis symptoms with constipation: the delayed gastric emptying from GLP-1 agonists can cause early satiety, bloating, and feeling "full" for hours after a small meal.

Tips: eat your protein FIRST (most important macro), then vegetables, then carbs last. Chew thoroughly — 20-30 chews per bite isn't excessive. Avoid carbonated beverages with meals. If symptoms are severe, your doctor may recommend a gastric emptying study.

Following on from lucas_SP_BR — and this may be the naive question:

Whether fiber actually helps GLP-1 constipation or whether the evidence is borrowed from a different population?

Closing the loop on my own question.

Update — the osmotic agent was the thing. I had been reaching for a stimulant, which was making the next day worse.