It looks like the way I wanted to approach my "super flow" may not actually be possible because of the way that Workflow supports regex or, rather, the limitations in its support.

It will work for a post with a single image but I'm not sure it's possible to replace one marker at a time on a loop - at least in the way that I've been trying.

Maybe I (and some others) have missed a trick in the regex syntax used by Workflow but what works when tested in places like regex101 does not work in Workflow.