I wondered if they were one and done messages, like in a business type scenario, where directions or other information is passed along with limited or no reply expected. That would be the best situation as creating smaller groups where it wouldn't be important for others to see the reply might allow you to simply send the same message 3 times to 3 groups. Since there is banter this would not be ideal.
If you normally initiate then the afore mentioned scenario again would not apply as you are initiating conversation and would like input from whomever.
If you reply to a message started by someone else then I would have to imagine that everyone is getting the message or there would be follow up comments with recipients being lost as to what the replies were to. Since that doesn't appear to be the case I guess that everyone receives your replies once the group has started.
This brings up another point, why do you constantly rebuild the group? If you are basically contacting the same people in established conversations, given the issues you're facing, reusing an established group that you've already successfully replied to would seem to be your work around. As the issue only appears to be an issue if you attempt to start a group.
I have several contacts with multiple numbers. Your contacts app will allow you to add multiple numbers naturally. I wouldn't think it would be overwhelming if your friends had more than one number for you. Since you brought up the WiFi calling and limited plan, it wouldn't be unreasonable for you to request calls go to your GV# and texts or messages could go to either. You would just be limited to using the plan number when initiating messages but if they add both numbers to your contact information they've likely already saved you would still appear as you in any conversation.