I appreciate the context that you have shared as that is very helpful! You can change the message that is being displayed by Changing the wording in themes. In order to do this, you will need to navigate to Online store > Themes > Actions > Edit language, I took a screenshot from my test store to show you the section I am referring to:
Once you are editing the language, type in what you would like to change in the search box. In this case, we can type shipping calculated and make changes to the section Taxes and shipping at checkout and Taxes included but shipping at checkout.