The 6D is currently around 30% cheaper than it was at launch almost two years. That makes it a great buy, you will only save more than that by going without it for longer. You Are already well on the way to having a set of full frame lenses, so could get good use out of the 6D straight away. Having both crop and full frame sensor cameras will also give each lens a dual identity. I have a 6D and 70-300, but am considering a crop body to give extra reach. You already have the crop camera, so would be buying in the opposite order.
I know that the usual advice is to spend money on glass, but in this case I believe from my own experience that you would be better able to assess your lens needs once you have the full frame body. Go for it, try out your 35, 50 and 70-300, then start thinking about future lens purchases. Having cut my SLR teeth with film at ISO 100 I am truly amazed by the performance of a modern digital sensor, and find that fast lenses are only needed for creative reasons. Once you have the 6D you can start to form your own judgements.
Whatever camera body you buy, you will be able to get something better and/or cheaper in a year or two time. When the 6D2 and 5D4 come along, the prices are likely to jump up at least initially - as the current prices of 7D vs. 7D2. When I bought my 6D, I could choose that or the 5D2 at similar retail price. That's not so true of lenses, but they are nothing without a body behind them.