There are many saas starter kits: some free, most paid. There are very complete ones which will safe you from having to do any saas plumbing like you say.
Great. Maybe the term "SaaS starter kit" is what I should be searching for. That's helpful. Thank you.
Here is one that I ran into a few days ago on HN: it seems very complete. I have no affiliation but it just seems more complete than others I ran into.