I have a shortcut to send meeting minutes to all attendees throug email. Basically I copy the minutes from my notes in rich text on a Evernote and I pass the notes to an email (after getting the event details from the calendar including the email addresses of the attendees) but they are passed as plain text.
It would be awesome to pass the text to mail as rich text.
You haven’t given any indication of your current approach or specific app destination for the final step, so I’ll cover things more generally.
Rich text on the clipboard can be copied into many different mail apps.
I favour having the content built as plain text with Markdown, then converting that to rich text and copying it to the clipboard. It gives me precise control.
URL schemes rarely support rich text which may be a challenge you have inadvertently been hitting. But do check actions for what sorts of inputs they accept. Rich text has to be mentioned explicitly for it to be valid.