FWIW, I’m on a tangential path. I’m preferring to export to markdown (heading to Obsidian). The best path is from Highlights either into Devonthink as a markdown textbundle or (as I’ve just discovered) directly out of Highlights as a textbundle folder (not as a markdown file).
My ongoing explorations are documented here. My developments with Devonthink have also resulted in an AppleScript recently posted in this thread.
As an aside, I find the annotation tools in Bookends iPadOS to be as good as or better than those in Highlights iPadOS but the exact inverse is true for the macOS versions of these apps.
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JJW