Having a 6D and a 7D, I could only add that any low light shots the 7D struggles with, the 5D or 6D will do it easy.
All the techniques you learned to overcome this with a 7D will allow you to extract even more with a 5D. Its worth it.
I still use the 7D for bird and action stuff when light is good and its handy to have a camera you can knock around a bit. Personally I don`t see the point in upgrading to a 7D II, I will consider 7D III, however.
Also keep in mind, the extra reach with the crop sensor is handy and you will miss it with a full frame sensor. Which is fine, maybe look at a 100-400L for longer reach.
The other good thing about having a crop sensor and a full frame sensor is that instantly you have much more flexibility with your lenses. For a bushwalk , I only need to take the 6D with the 17-40 and the 7D with the 70-300 and got wide range and long range covered. Putting the 70-300 on the 6D covers medium range.
When I go kayaking I just take the 7D with the 17-40 ( its light and small ) and the 70-300. If I take the 6D I put it in an extra waterproof bag.
The 6D is a confident, light and small camera and its centre focus point is even better for action than the 7D and in comparison it can see in the dark. The outside ones not so much, but with good technique they still work well. Certainly not much behind a cross sensor on the 7D. My wife really enjoys using the 6d as its a nice size and not too heavy.
Having both may suit you, just don`t let the 6D get too wet, its not as robust or weatherproof as the 7D.
Having two bodies also means having a camera always ready with the right lens, its a huge pain having to swap lenses all the time.
Maybe keep the 7D and add a 6D and the 17-40 or the 24-70 Tamron or 24-105 and sell the 10-22 and 17-85. The 7D really works nicely with the 2.8 Tamron 24-70, its actually a useful range for portrait or medium range action stuff. I wouldn`t bother with the 24-105 f4, its to general unless you want to add a prime or two and the 7D really benefits from the larger aperture.
I wouldn`t be without a crop sensor and full frame sensor now, between the two I can cover 99% of what a 5DIII would give me and I would find it hard to justify a 5D III over a 6D and I don`t have to swap lenses over all the time.
Just my thoughts and what works for me.
Canon Powershot S95, Canon 6D,7D, Canon 40 2.8 STM, Tamron 24-70 2.8 VC, Canon 17-40 L, Canon 15-85, Canon 85 1.8, Sigma 30 1.4, 50mm 1.8, Canon 100 2.8L Macro, Canon 70-300L +Kenko 1.4 Pro 300DGX, Canon 430EX II and RS 4 Classic