I setup a Shortcut to email to Nirvana. I have it set as a complication and it takes two taps to run then another to stop dictating. I'm not a Shortcuts expert but I used Dictate text then Email with my Nirvana address set as the recipient and set the email subject to the Dictated Text. I couldn't find an answer anywhere so I hope that helps! It will also work with your iPhone or iPad with Hey Siri but you have to click to send the email.
Before that, I was using the excellent Drafts app for quick capture.