I DIDN'T take my 990 KTM on the TAT ( took my 525 KTM) because I realized that if I sunk the rear wheel in sand or mud it would be an ENORMOUS chore to get it unstuck. With a smaller bike I can just grab the bike and lift it. DRZ is a better choice the the GS IMHO.
I started with a kenda 760 rear tire. It was completely used up in 1400 miles. Both of my riding buddies used dunlop 606s. We did the western TAT, about 2500 miles, and they both had lots of tread left. 606 or a Maxxis IT would be my tire choice. Maxxis get better traction then the 606 but the 606 last longer and is more than acceptable.
I think that you are making a mistake with your gearing (39 tooth rear). Once you are set up you need to find a long (1 or 2 miles) section of DEEP sand , like a river bottom, and see how it works. It is easy to burn up a clutch with too tall gearing. We all used the cheap ( $15) steel sprockets from rocky mountain. They went the distance with hardly any noticeable wear.