After much tweaking I've pretty much created my own custom templating engine for Letter for dynamic content import based on Handlebars (https://handlebarsjs.com/)
Ever since the earliest version of Letter, 'content import' has been a key feature. You can see it in the earliest video here: https://www.producthunt.com/upcoming/letter/messages/emailotter-video-update-press-play.
A user inputs article links they want to import, and the email HTML template is populated with that content. This was okay, but it meant edits to plain text areas of a template were lost! More on that here: https://www.youtube.com/watch?v=dKqlVH52SqY&t=5s
I knew this was a problem, and have been constantly tweaking how it works, and ended up with something new. I never imagined it could actually work the way it does today when I started out..goes to show that things you may think are impossible may be possible.