If I want to create a managed hosting platform service, where should I start?

If you don’t want to deal with servers and stuff, but offer something unique, you could look into using a white label solution.

Be aware though that the hosting game is a though market to be in.

I saw the following strategy working in the past (short overview):

• Set up a landing page with the offer. Let the client fill up with essential info about the site they wanna a build. Also, they pick one of the hosting providers you suggest. All of them can be an affiliate partner. At this point, you are doing everything manually zero automatization on the client-side. You are handing over access after you are done. This is the longest part.
• Next step, depending on how you decide you automatize the deployment on the client's infra (their account on a selected cloud or hosting provider) or on your infra (infra you manage or run on top of a cloud)
• The last step is the automatization on the frontend side, payment, subscription, etc. At this point, you should have a good understanding of what offer resonates with your target audience since you did the work for a long enough time.
  Regarding tech stack, for the late-stage, I would pick an IaaS with excellent Kubernetes support and automatize everything with Terraflow.