Please check below screenshot to enable media volume sync. You will be able to control your bluetooth device volume and phone volume separately. Go to developer options and enable the "Disable Absolute Volume" and then restart your phone and you have done it.
Through the above mentioned option you can manage volume separately of your bt device and phone. Please check attached screenshot for your reference. Before enabling that option, the volume of my device is somewhat low even increased at full volume. So whenever you enable that option, you will be able to increase volume separately, loud and clear. This is the purpose of the option.
