Try turning wifi off and see if it works.
I have the exact same problem on AT&T and it seems when wifi is on along with advanced messaging, the app can't connect to the RCS server to upload photos. Turn off wifi and all is fine. My wife's note 9 is doing the same.
Same thing happened to me about 2 days ago. I did everything you did and I even did a factory reset and it didn't work. I also turned off my WIFI to see if that helped but it didn't. I can't send MMS OR receive them. I'm wondering why this would be a ATT (network issue).
I recently just received my Note 20 Ultra and it's under the Tmobile/Sprint carrier. I cannot send picture/MMS to certain phones. My other half has the same phone as me and I am able to send him picture/MMS messages. When I try to send to my friends, iPhone users, the notification of not being sent pops up. What is going on?