SMS/MMS Not Always Delivered/Received

Not sure why my original post was deleted, so I'm posting this again and hoping for some help!

Bear with me if I'm a little long-winded here, but here's a TL;DR: For months now, I've had intermittent issues with receiving and sending MMS and sometimes SMS on five different Pixel devices. 

The story: I noticed this on my Pixel 2XL months ago when my family members (both ATT iPhones and Android phones) would send me MMS messages that I'd never receive. Mind you, they're not even incoming/undownloadable; they don't show up at all. I'm lucky enough that we keep in touch often over other apps, and so I'd always get the "why are you ignoring me" or "you're ghosting us!" comments. Since then, I've gone through 2 RMAs for my Pixel 2 XL and 2 more RMAs for my brand-new Pixel 3. I'm on my third Pixel 3 and my second brand-new sim card, but I'm still running into these issues.

The issue: It is, unfortunately, extremely intermittent and random. Sometimes I'll send an MMS successfully, but then I hear that my message never got delivered on the other person's end. Or, sometimes I'll try to send MMS messages but get stuck in a "sending" loop, where I have to toggle Airplane mode on/off to be able to send the MMS message. It's usually around these same times that other messages drop or mysteriously never get delivered. I've checked APN settings but ATT has never suggested I change anything from the stock settings. I've changed sim cards. I've tried everything on/off Wifi. I've tried ATT/non-ATT people. I've tried Android and Apple phones. There doesn't seem to be a pattern. I have seen dozens of people on Reddit and on forums complain about similar issues with ATT/Pixels. 

This is a huge and disappointing issue. Like most everyone out there, I rely on SMS/MMS often to communicate with loved ones, clients, etc. Having to constantly check with people and ask them if they "got my message" is embarrassing and not realistic. Not knowing whether my messages have even been delivered (or if I'm receiving other people's messages) is even worse. 

Every single time I've called ATT, they've said, "It's a non-ATT phone, we do expect issues especially with Google." I don't understand how this carrier can say something like that and completely pass the problem along to Google. Google's been more helpful and has constantly troubleshooted with me to no avail. I've also had multiple tickets opened by ATT, the latest being an "urgent" issue where I would get feedback within 24 hours. Of course, I haven't heard a single thing.

ATT/Forum members, I'm begging for some help or advice. I've been part of ATT since the Cingular days and I really don't want to leave. But, if this continues, I have no choice but to switch carriers for the entire family. I can't use a service that has unreliable sms/mms in this day and age. Any and all advice is appreciated, and pre-thanks for your time.