Good afternoon Kyreeone,
Welcome to the Apple Support Communities!
I understand you are having trouble creating a group in the Messages app on your iPhone. The resource below explains how to work with the various group types. As you have discovered, you can't add or remove people from group unless they are all on iOS. I've highlighted some key information here for you. Click on the link for complete details.
Send a group message on your iPhone, iPad, or iPod touch
These messages appear in green text bubbles and go through your carrier instead of Apple
These messages also appear in green text bubbles and go through your carrier. Group SMS messages don’t support multimedia attachments, like photos or videos. All responses in a Group SMS are sent as individual text messages and the recipients can’t see the other responses from the group.
Anyone in a group iMessage can add or remove someone from the conversation. You can remove a person from a group iMessage that has at least three other people.
You can’t add or remove people from group MMS messages or group SMS messages.
Since you have already created a group in the Contacts app on your Mac, the only solution I can think of is to send email messages to this group instead of text messages.