Has anyone had experience productizing digital marketing services into a well-coordinated marketing sequence/solution?

Do you mean the marketing of such a service?

Or the fulfillment process itself?

(Yes, to both.)

You'll probably benefit from looking up some Michael Gerber / EMyth content.

Essentially, you can systemize your process, while retaining the "secret sauce" that makes what you do special. You just have to isolate what that is and ensure it's included in every project.

Yes. Don't really know what else to say but have to say something so that this answer is long enough to get approved.

Yes. I've done this for musician clients. There are several online tools you can utilize (many of them transaction based) to beta test your solution before you develop it. For example, you could launch a course as a product, and try it out on your lead base before your develop a true platform. The obvious key is converting people to your process, so working on the kinks with a course-based launch is a good way to go. You could google inbound for months, or call someone who has already done this successfully... :)