[You could create a template message using Shortcuts then add a voice trigger. That would be actionable from the Watch. The limitation would be that it would basically be a non-interactive message for each trigger, e.g., telling your SO that you are on the way, but you couldn’t then use the same trigger to send a message to your boss or a friend instead.
You can build customisation with Shortcuts, but interactions only work on the iPhone / iPad.]
Sorry - misread your original message. You are on the iPhone
Try building it in shortcuts, I have one that works out how long it would take to get home then sends that info to my wife.
Another thought - have a shortcut that loads the message into your clipboard then just paste that into the message field. It depends what aspect of the process you are trying to automate.