It is entirely viable to never have more than 1 or 2 open pull requests on any particular code repository, and to use continuous delivery practices to keep deploying small changes to production 1 at a time.
That's exactly how I've worked for the past decade or so.
Stacked diffs seems like a solution to managing high WIP - but the best solution to high WIP is always to lower WIP
Absolutely everything gets easier when you lower your work in progress.