Life story, but maybe it can help you... I have been having a similar issue for a month now (12/25/20, which is also when i upgraded to IOS 14.3). I was getting a not delivered message on texts, mms, and group texts with android users. I reached out to my carrier (Tmobile) originally and they did not see any problems in my area. I worked with them for a while because I was also having data speed drops. They sent me a new SIM and cellspot and it did not correct the problem. I was still having the not delivered messages and dropping of data speeds, while my husband was not who would be sitting next to me. My carrier then sent me over to Apple finally on Saturday (1/23/21). I am now with a T2 tech. He could not find any obvious issues either so on Sunday (1/24/21) he called me back, and walked me through a factory reset and is touching base with me again tomorrow (1/27/21) to see if it has been happening. So far in the last 45 hours, the problem has not occured since the factory reset. I understand if you don't want to jump right to that though. I have sent him numerous posts on here of people having similar issues, including yours, because I personally think this is a bigger apple problem.