I appreciate you posting your question here on the Community page, Mark. I'm here to help you with your query today.
Doing a mass text using QuickBooks Online can be made possible with third-party apps authorized and integrated within QBO. You can browse for apps in QBO that can do a mass text.
Here's how:
On the left menu, go to Apps.
Click the Find Apps tab.
Search for tools that have the best feature to do mass text.