Hi, is there a way to run Deployments (e.g. pulumi up) on one stack at a time with custom validation in between each deployment?

This is the normal flow. Change to a stack, deploy it, check it, change to the next stack. What behaviour are you encountering?

Sorry if this is a newbie question - if I had a GitOps workflow w/ multiple stacks, and multiple stacks have changes - how can we make stacks be updated 1 by 1 w/ validation/sleep in between each one?

A loop. You have to use a loop: there is no way to update multiple stacks with a single command.