thiogs like number of satellites tracked/used and 2d/3d would be very helpful. Much more is available when you get away from NMEA.

Well, isn't 2d/3d what the "mode" in the GSA sentence means?

And of course in many cases you can have lots of stuff if you're willing to deal with proprietary sentences. The Rockwell Zodiac has goodies that you can have if you're willing to deal with PRWIZCH for instance (and for Earthmates, gpsd spoofs NMEA from the binary data)