The 70-200 is my go to lens as well, love that thing! The 500 f4 was a beast (I ended up dropping a couple of grand on just a tripod setup for it just to hold the weight and make it useable is some scenarios. lol) and was super sweet. I used it all the time with a 1.4x TC on my 20D. That gave me something stupid like 1120mm @ f5.6 or so. I got some pretty sweet shots of the moon with that setup. I kind of regret in now, but I sold that lens about 10 years ago :( I used to shoot for some automotive magazines and also covered quite a bit of motorsports, so that lens was pretty much a necessity. But once I got out of the biz, it was just a very expensive lens laying around collecting dust. It was great for wildlife in my personal photography, but nothing the 70-200 with the TC could not do really.
I really like the 6D in the image quality department and it is very nice for astrophotography. In fact, it was the best camera Canon made for that type of photography when I bought it (I think it still might be). There is just something about the sensor and ISO settings that really work well in very low light. It has very little noise in the upper ISO ranges. I also wanted a full frame. The only thing I don't really like about it is the form factor. Between the 20D and getting the 6D, I had a 1DMkII as my main body (all the action stuff I did) and while big and heavy, that think just fit my hands perfectly. It simply "felt right" when I picked it up. Even though the image quality is not quite as good as the 6D, I still tend to use it unless I need the full frame or ISO of the 6D. I would have loved to get the current 5D at the time, I could not really justify the cost since it was going to be used for such a niche subject like astrophotography.