Most (all?) of the existing BT tethering apps are broken on Marshmallow, and none of them seem to be getting updated, so I wrote my own.
The first time the app is run, it will open the system settings permission dialog to request the WRITE_SETTINGS permission. This is required under Marshmallow in order to change the Bluetooth tethering setting (and I assume is why all the older apps have stopped functioning.) After allowing this permission, run the app again to enable BT tethering.
The app has no real UI, the activity opens a blank page, enables tethering, then closes again.