Greg
What about the GM 100-400 and using the 1.4x when needed? Maybe it's lighter-I must check. Plus the 200-600 is not a GM lens. Also one must say to himself how often will I actually use that lens. I used my GF 250+1.4x a lot on those same DV Dunes and just cropped where wanted, but it was not often needed to use it or crop images. There I had the 50R with 23, 32-64, 100-200 and the 250 with 2 bodies. Not that heavy for me. I use a Mindshift 26L TT bag where I can just swivel it on my waist to get into it if changing bodies/lenses. Normally kept the 32-64 on one body and the 100-200 on the other.
You might want to consider a 2 body approach to minmize lens changes and it serves as a backup in the event something happens to one body. I had it happen once and vowed it would never happen again especially since I was traveling many miles from home without easy recourse.
If you keep this up you will get over 15 lbs in a hurry!
Just saw the 150-500 suggestion. That might be a very good option against a Sony G lens and would not leave you much of a gap between your 24-105 and the 150-500 FL whereas the GM 100-400 leave no gap, but has probably more weight. Decisions, decisions.