Thanks for the suggestions! Was really sure I was overlooking an option in the Calibre book editor. But perhaps not. I've been going back and forth between Sigil and Calibre's book editor until I decide what my priorities are. Sigil is bit more straight forward, but Calibre is more functional and powerful. My regex knowledge (and memory capacity) was from 20 years ago so it's been really slow building it back up. So sad. But it's useful and good brain exercise.
Really appreciate the help!