You're also adding an extra build step that by its nature can't run in CI since it generates the CI pipelines. So now you need some way to keep your Dhall and Y

You're also adding an extra build step that by its nature can't run in CI since it generates the CI pipelines. So now you need some way to keep your Dhall and YAML in sync. I suppose you could write one job in YAML that compiles the Dhall and fails the build if it's out of date, but it seems like a lot of extra work for minimal payoff. Instead, if you want to stay away from YAML, I'd say just move as much of the build as possible into external scripts so that the YAML stays very simple.