You know, none of that worked. What did work was turning off stupid Wi-Fi calling! I realized it may have been related to this when I was able to send MMS when I was not at home. I'd been keeping Wi-Fi calling on "WiFi preferred" in hopes that WiFi calling would actually work - it hasn't - because my cell reception at home is not good. But it turns out that has done more harm than good!
This probably keeping your phone from (briefly) disabling WiFi to send the text. We have two older phones in the household that you have to be careful with (an HTC M8 and a 1020), if you leave WiFi on, and send a MMS text (attachment or group), it gets into a weird loop, where MMS cell data is shut down completely.
I can usually get the phone to reset the cell data for MMS by going in/out of airplane on these phones, sometimes several times each (the M8 is easier than the 1020), and also by sending the respective account a MMS text to itself (usually just a pic).
It's a pain, but the ONLY issue that the 1020 has had; the M8 has hung a couple of times. Both run fine, other than this oddity, but I think this is more of a general OS issue that will get fixed, eventually...
We saw this MUCH more frequently on T-Mobile, but occasionally on AT&T now too. I think this is due to net congestion on T-Mobile, but that's just a guess, it gets into a similar loop where it thinks MMS data isn't working, and the OS goes sideways...