I never used bt tethering, I always used a forwarded bt pan connection.
I got it working with devuan ascii, but laptop is at home and there are some differences compared to debian.

I will give you a link how I did it in the past. However with devuan ascii it is different and I will have look and try to remember what I did.

In principle you need to initiate a pan connection the rest you do with genwall.

http://talk.maemo.org/showpost.php?p...5&postcount=62

Pan and modern bluez:
http://blog.fraggod.net/2015/03/28/b...-bluez-5x.html