I created something very similar but to display raindrop links into the remarkable and sync highlights back into raindrop. I also added a GenAI powered paper summary filter as a preface to the papers it send to the remarkable which is working quite well.

As you mentioned Remarkable file format is a PITA to extract highlights, one thing that helped a lot was to add an OCR fix phase that uses Gemini flash model to fix common OCR errors and to merge single highlights that are across pages.