If you have Alfred, you can easily have dynamic inputs in snippets, see:
https://www.alfredapp.com/help/workflows/triggers/snippet/snippet-triggers-with-dynamic-inputs/
I use these all the time. You can even chain multiple inputs, storing your values as variables along the way, to paste a longer message with multiple custom arguments.
I also use Superhuman for email, and that allows dynamic snippets on emails for iOS (including filling first/last names) - which is even better than TE because it runs without switching between apps. It makes emailing on iOS on par with desktop.