Thanks! Yes, build the site for a single fee. I like how you put it! Might just steal that copy for the website itself haha.
I use a google form to onboard the client to give me all the copy, content, images, logo etc, using annotated screenshots of my own site (because the site itself is a demo) to guide them on what to do. If you click on the "buy now" button, it brings you to the form. It's not ideal - like I wished they can upload files directly, but havent got that figured out yet. Referring to images/site back and forth might not be the most intuitive for customers too, so will get feedback and iterate and see. But yes, going niche and productized helps because it's a very fixed set of specs or features, little variations so it's easier to show them what I need.
Re: finding niche, it wasn't really scientific tbh, as it's an MVP at this stage, so just throwing this out first to see if it sticks. I just went with my own experience running my own productized consulting site and realising that a service like that would help (instead of tangling with WP, SQL injections security breaches, and a constantly updating version).
And also a bit of experience talking to and working with peers who are also running businesses and need sites. Every site builder is asking them for a few days or weeks of learning some new CMS to build the site they need, which many are not really interested to do (either no time, or interest, or just not talented that way in design). They are not in tech, not inclined to, e.g. one customer is running a user recruitment service, another in events management. They are used to outsourcing and prefer getting a designer/dev to build a site for them at a fee, but as small biz they don't have the thousands of dollars for some good freelancer or agency.