A nice editor for html emails.

Use something like: https://github.com/unlayer/react-email-editor
Or tinymce

Need to substitute in smart-values.

Possible Workaround

As a workaround in the meantime, we do have customers who use html emails. The way they have done it is copy the html produced by Jira's emails and then substitute in the new values. Makes for hard to read configuration but it is possible.