Oh man, I know that's super annoying but you've done awesome with the steps you've taken already. I'm thinking this could be an app too. Have you downloaded any 3rd party apps and then noticed the error? You could try uninstalling a few of them then giving it another shot. Try flipping the phone to Safe mode and see if they same thing happens. Doing that will disable 3rd party apps temporarily but it's a great test to see where this issue's coming from.
Any luck with putting the phone in safe mode?
I have a Galaxy S9. I turned off the permissions to SMS on the gear s plugin and the error messages stopped.
We have a similar thread going on over here Note 8 IMS Service has stopped error over and over error. There are some pretty good tips for folks that run into this error even though it is not for the same exact device.