Not sure if this is off-topic, but can anyone recommend a CD tool that’s good for authoring/managing pipelines made up of many independently-deployable Pulumi Stacks?

Have you looked at writing a top level script in your preferred language to use the Automation API? If that's not going to help you might have to provide some more details. My whole pulumi code base is designed around multiple projects Networks, databases, Resource Groups etc

Thanks for the tip Peter, will look into the Automation API.