Automated text messages?

I am looking for a way (or an app) so that I can send a text message to a select list of contacts. The message to each contact is the same. But I do not want to cut/paste the entire list of phone numbers into the Messages app (which would create a group text thread). Instead, I want to send the text message to each phone number individually. So pretty much a standard "mail merge" that you can do in Microsoft Office... except I want to send via text message/SMS (I am assuming the Messages app is how I would send).

Just for context, I am a photographer and we shoot team & individual sports. When the photos are posted online, I want to text message each athlete that the photos are available. The athletes have given us their phone numbers and permission to text them as part of the registration process. So nothing shady going on here! ;-)

I know there are services out there to do this. I'm looking for a way to do it myself on my Mac.

Any suggestions?