Possible solution!
The last few days the volume for texts being recited through BT connection has been at the same level as music media being played on my infotainment center. Not "quiet."
While playing your music at normal volume level, have someone send you a text. When you get the announcement of an incoming text and the option to listen; tap yes and QUICKLY turn up the volume before the text begins to be recited.
Since doing that, I have not had to reach for the volume knob when texts have arrived and were recited.
It has only been a few days, but I do believe that has solved the "QUIET" volume problem coming across via Bluetooth.
Try this, and let us all know if it works for you. Hope it is not just a fluke.....