Thanks for the feedback. What I ended up doing is creating the production environments by hand, and then reducing the render.yaml file to just describe the staging environments. With Autosync and preview environments enabled on that staging only blueprint I have mostly what I’m looking for: New PRs against staging create a single preview environment and I can specify the exact names of the environments. Merges to master don’t get preview environments with this solution, but I can live with that as I don’t do that very often.
A key change I also had to make to the render.yaml file is removing the branch: lines, because when those are present the new preview environments will use those branches, not the new feature branch that I want to test.