Did you try to delete your phone from the car's system and add again? (also, delete the car from the phone's BT settings) I have no issue with the BT sync with my ford explorer's my ford touch system, before or after update. I just checked with my friend, he has no problem syncing his S6 (6.0.1) with his BM Z4. Both of us got SM-G920F
It is very difficult if not impossible to revert to previous OTA update as the bootloaders are modified to block any previous version. But now there should be monthly security updates, so expect Samsung to do some bug fixes. Do give a feedback to them.