I had a similar question recently.
There are some contacts whom I don't have blocked, but whose texts aren't a priority. I would rather not see the notifications while driving, or have them interrupt whatever I'm listening to on my 4"-screen Sync system.
Unfortunately, the only option I could find was to disable text messaging through Sync entirely. From what I can tell, it must be done through the phone.
For example, on my Motorola x4 running Android 9.0, I must go into the phone's Bluetooth devices, settings for the "SYNC" device, and turn off the "Text Messages" option.
There does not seem to be any way to get fine control of notifications, that is built into Sync itself. I'm not sure what a factory reset of Sync would accomplish here.
FWIW, I am using the default SMS app on Android, Messages. You can either block a contact from texting you entirely, or choose not to show notifications for that contact's texts. The latter means that you don't get a popup or lock-screen icon showing a new message. However, it seems that Sync is ignoring or unable to interpret that setting. So, I was getting Sync notifications even for the contact that wasn't generating notifications on the phone. Annoying.
I'm OK with not texting through Sync. If I really need to text someone I'll just pull over/park somewhere. Or just make a call, Sync works very well for that.
