You can disable MMS Messaging so it uses SMS instead, then the recipients shouldn't see it as a group message. At least one recipient will need to be a non-iMessage user so that it sends using SMS instead of iMessage. You should be able to tell how it's going to send based on what's displaying in the message box...either 'iMessage' or 'Text Message'. You'll want it to say 'Text Message'. If all recipients happen to use iMessage, then temporarily disable iMessage on your end before sending the message. You may want to try this out with a small group of people first to make sure it's working as intended.
https://www.reddit.com/r/ios/comments/18ehdm1
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Another option would be creating a Shortcut to do this. If you're familiar with Shorctuts, you could make one so that you enter/select a list of contacts, type your message once, then loop through the list and send the message to each one individually.