I use Ubuntu Mate and other than needing to force a connection on the N900 by doing something like checking email(forces the net connection to active which PAN doesn't always do for my LineageOS and formerly Ubuntu Mobile (Android hybrid)). My laptop usualy can force the N900 to go to 3.5G.
I use the N900 as my network AP whenever I am working mobile or out of the range of my WiFi AP.
There is IMHO no reason to use anything other than USB networking or BT PAN when forwarding the N900's (or most any other mobile device's) network connection. Wifi is unwise as far as the battery mA/h waste vs USB/BT but it often requires less effort to set up for a novice friend's device.
I think the BT PAN needs to be DEB-ified and moved to the repos or even a CSSU update as it is such a core functionality for one's everyday mobile phone I can get web browsing form a tethered tablet but if the N900 remains an EDC phone it needs to support basic tethering.
Which debian GUI program is managing your network if you are not using something like dhclient to dhcp and whatever is equivalent to iwconfig for BT to connect.