Hi everyone,
I've read this entire thread, and many others as well. I've tried all of the suggestions in this thread to no avail (delete phone from car, delete car from phone + repair, reset all network settings, hard reset (even set up phone from scratch; no backup restored), installed latest 11.3 beta 3).
However, today I came across this thread: bluetooth keeps dropping and reconnecting
It's a very old issue (since iOS 7ish), but it appears to be the same and hasn't been resolved either. They have a particular workaround I have yet to try, but many are reporting success in there; turn off WiFi.
I don't want to get your hopes up, but I'll definitely report back in a week on whether it has helped or not. In the meanwhile, I highly suggest you guys try this as well, and report on your what kind of success you're having with this workaround in place so that we have more data points at our disposal.
Best of luck,
Gary