Moving one WordPress site is easy. Moving twenty in a weekend is a pipeline, and the failures that cost you are the quiet ones: writes landing on the old server, cron firing on two boxes, certificates blocked by ACME rate limits. Here is the fleet migration process I use, from manifest to per-site rollback.
A plugin quietly reverts after every deploy and nobody knows why. That is what happens when Git and wp-admin both think they own the file tree. Here is how to build a deployment workflow where the repo is the source of truth and the database only ever travels one way.
A deploy key in GitHub secrets is a shell on your production server, handed to a container you don't control, running code from maintainers you've never met. Here's how to build the pipeline so a leaked key isn't worth much: forced commands, scoped sudo, pinned actions and a real approval gate.