Believe I recall a thread from a year or two ago similar to this. User had an app that setup group lists and then sent individual texts via Messages. Sorry, don't recall the app names they were using.
If adventurous, might be able to do this via a Shortcut. Idea: setup a Contact group with the contacts in question, find the contacts in the group, cycle through each contact and send a text. There is a "find contacts" method, and a "send Message", and a "get contact details" method.
Might be a Shortcut in the gallery for that as well.