After reading this (and because I'm not a developer who could use this to make my own plugin), I searched to see if there was another similar plugin. I found one that's very nice, easy to use, and very similar, called Pre-Publish Checklist. It includes the interface to add/edit the list that's easy and straightforward to use. I was able to get mine setup and ready to go in a few minutes.

It's too bad they don't include a screenshot of the checklist (in the Block Editor), but it looks quite nice, and fairly similar to this one. Plus has options for what to do if the list is incomplete and someone tries to publish the post: block publishing, just show a warning, or nothing at all.

Note: I'm not associated with the creators of the plugin at all. I just opened this article hoping to find a solution to a need. When this proved an "almost" solution, I went searching for one and was really glad to find one that works great.