Upload & Sell: Off
My thoughts based on your current selection:
The 400 5.6 is considered THE standard for budget vs reach. There is so much debate here and other places about how a 300 plus 1.4 or any of the 200mm plus a 2x compares. I am looking at the 400 length myself, so I have read most of those threads. The first big constant is that NOTHING will match the 400 prime for IQ in a lab test. It is the king of the hill. The second is most every other combination has IS, which the 400 prime does not, so if IS is a requirement, there is your answer. The 1.4x degrades much less than any of the 2x extenders from what I have read, so the 300 plus 1.4x is generally considered #2 after the 400 prime.
You say you did not like the 100-400. I have heard the sample to sample variation was/is quite wide, and that a good copy can run IQ wise with anything out there and even brush up to the 400 prime IQ. The problem is nobody with a good copy wants to sell!
I have heard mixed reviews of the mark II 70-200 f2.8 with a 2xmkIII, so I plan to rent the 2x and test it within the next few weeks since I own the 70-200 already. If this produces satisfactory results, I will have a 140-400 f5.6, great versatility! I could have a $500 solution to a $1200 problem... You have the f4 zoom, so this would not work for you. I had the f4, a great lens! Sold it to fund the 2.8 since I needed 2.8 for indoor sports, otherwise it would still be in my bag!
So, how important is IQ? Do you need IS? do you need 300mm? A few things to consider