Aside from the Automation API, is it possible to create a stack programmatically within a project while still using the CLI?

No this isn’t possible

The reality is I should probably just use the Automation API, but I’d essentially like to use it to create “tenants”. I’m mostly doing this progressively, so first doing everything with the CLI for a single tenant, and then I was hoping I could parameterize it via the CLI (and support multiple tenants) dynamically, and then eventually convert it to the Automation API.