Last week I made 50+ of my own newsletter components for the template library of Letter.
I knew HTML email code was horrible, but didn't quite realise how tricky building them are. Making sure they work across all email clients is a pain, especially for Outlook 2000 on Windows!
You can see all the different components here: https://twitter.com/graeme_fulton/status/1228499025026002945
I found that it's really important to have the best starting point for users in your product as possible. If they arrive to the app, and don't know where to start (not enough templates), they're probably more likely to abandon the product.
Previously I was depending on using open-source newsletter templates, but found I needed more variety in templates to help people start better. And that meant really learning how to make a responsive HTML that doesn't break in MS Outlook...painful!