You didn't provide enough information for anyone to really give you a good recommendation.
We do not know what you shoot. We do not know if what and where you shoot if the size and weight of a 600mm set up could be used. We do not know what will make you happy.
I'll second that! You say that you don't have enough reach. OK, how much do you have to crop to get the images that you want and what focal lengths would give you those images?
For example, the first time I went to Watkins Glen, it was with an EOS-3 film camera and a 100-400. Looking at how much I had to crop, I decided that I needed 800mm or its equivalent. Now I use a 7D with either a 100-400 or a 300 f/2.8 with 1.4X and 2X TCs. By next spring, I'll probably own a 200-400 zoom.
FWIW, Sports Illustrated photographers typically prefer the 400 f/2.8 with 1.4X and 2X TCs over a 600 f/4 with a 1.4X TC.