BTW, I’d love to see your approach here. Are you putting the CSS and other elements directly in the note, or are you using the templating approach?
Have you seen our meetup for earlier the month on email management, while not a letter the methods are similar: Tinderbox Meetup Sunday, 15 Sept 2024 (Video): Emailing, Templates, Includes, CSS. Also, if you integrate Pandoc into your workflow, you can easily export you “letter” to a word file.