Canon Big White Conundrum - 200-400 vs 400 f2.8 IS II vs 500 f4 IS II

I am an amateur, and have purchased a 300 f2.8L 11, 500 f4L 11, 600 f4L 11, and a 200-400L f4 over the last 18 months, during which time I have been shooting lots of surfing on Queensland's Sunshine Coast (with my 1Dx). I don't have the 400 f2.8L 11!

In my limited experience, the 3 primes are incredibly sharp wide open. I don't use the 300 very often, as it is usually way to short for surfing (though I took it on holidays to the Norwegian fiords last June and loved using it and the results!)

I find the 500 and 600 both work well with the 1.4x 111 converter (again shooting wide open at f5.6), with a very slight drop off in image quality, though focus tracking on surfers is still very good and accurate.

The 2x 111 converter is a different story. Image quality on fixed objects is useable, but focus tracking accuracy on surfers drops off drastically, to the point where I have given up using it on the 500 and 600.

I have not used the extenders on the 300 as I don't need to.

I purchased the 200-400L earlier this year specifically to take to Africa next year as a "one lens" solution, but I need to experiment more with the extender engaged. It is as sharp wide open at 400mm as the primes, and focuses beautifully tracking surfers at 400mm (though it is often too short for this). The image quality again seems good on fixed objects at 560mm wide open, but results tracking surfers at 560mm have been less than stellar. I intend to persevere with it with the extender engaged (as soon as the surf picks up again) and will update my thoughts.

Weight: I can hand hold the 300 easily for long periods; I can just manage the 500 for short periods, but I struggle with the 600 and the 200-400; I always use the 3 big ones on a stick so weight is not an issue!

Many thanks and I hope this is of some assistance!