I have yet to find volume control possible with the TV remote for the usual setup of optical out to some bluetooth transmitter. I can't guess how you accomplished this but hey, I haven't seen this work for decades.
As to the factory reset there are caveats here. In a few TVs with analog out I have to find the menu for (and mind you not all TVs have this) that output and change the TV audio to Stereo or PCM along with another that usually reads something like "fixed level or vary with remote."
Since I have no access to this model I can only suggest the above and if Sony can't return it to how it worked proceed with a full return and refund as they broke it.
I have a Bravia 65X900E. I have a Bluetooth transmitter linked to a set of wireless headphones which work great until this last Sony firmware update. Now my volume on the tv itself is locked and cannot be adjusted at the tv or remote either one. My transmitter is connected with optical. I have tried to change every audio setting on the tv but can’t seem to correct the problem. I even did a factory reset and disconnected everything and powered off but didn’t work. Anyone else experienced this problem since this last update?