The only thing that I've found to work reliably is same-sized stories, i.e. breaking down every story until you get very thin vertical slices of roughly the sam

The only thing that I've found to work reliably is same-sized stories, i.e. breaking down every story until you get very thin vertical slices of roughly the same complexity (not very complex at all). The refinement should be done with (some part of) the development team and the product owner present. Ideally each of these stories is at least slightly useful to a user (if not it's hidden behind a feature flag until enough related stories are shipped to make it useful), but it should be testable in production. I also find that when you do this, sometimes the product owner realises that some of the slices aren't particularly valuable and they can be deprioritised.