If you want to increase your Android device Bluetooth volume, then go to your Settings.
Then move to the "About Phone" option and click on "Build Number" seven times.
You can go straight to the "Developer Options" in your phone settings.
Now scroll down to the "Networking" section.
Now enable the slider next to “Disable Absolute Volume”. Doing so will separate the volume system of Bluetooth and the phone system.
You can maximize the microphone volume Android levels respectively as per your choice.