Your response about my M1 mac is very interesting, especially since I purchased it and used iMessages successfully 3 months before I got my iPhone. Do I get a ghost iPhone when I purchased my M1? Ahh that explains it then I definitely don't know what carrier that phone is on.
But seriously, the problem is with iPhone pure and simple. Luckily most people in my circle use Signal, otherwise I would be at a disadvantage. I have T-Mobile, and housemate who uses Verizon is now having the same problems, only when sending photos from his iPhone to Android phone. I personally believe Apple should be forced to make their services actually compatible in the U.S. like they are being made to in other countries.