You are correct - these long lenses can be very hard to remove vibration. Some are more of a problem than others. I have a Tamron 200-500 that is an extremely long lens. Without a hand across the barrel, even a cable release does not produce sharp images unless there is a long delay. By comparison, I have a Nikon 300 f/4 that seems remarkably stable. The length and weight of the lens, the tripod head, the tripod legs, and technique all add up as sources of error.
I find that a hand over the lens barrel is usually better than a beanbag. I also use a hand under the lens barrel at times.
Live View is great for focus, but there is still mirror slap at the shutter release. If you have the opportunity, focus using live view then use Mirror Up or Delay to control the shutter release and mirror.
Distant subjects magnify any motion issues. Heat can cause the air to shimmer and create thermal effects that reduce sharpness. Try testing on subjects within 100-150 feet.
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