I owned Pentax 6x7 cameras (including the last model - II) in the past. I found the same problem you have encountered, even with non-telephoto lenses. I was able to mitigate the problem by triggering the Mirror-up and then placing both hands on the top of the camera (mounted on a tripod) and pushing down steadily, using a finger on my right hand to gently trigger the shutter release. This gave better results than using a cable release - very often tack sharp as seen by viewing transparencies with a high power lupe. Apparently the vibration from the shutter is significant. The longest lens I used was the 300mm, but mostly I used the 200mm and the technique I described was very effective - even for exposures as long as 1/2 second. I'm not sure about the 300mm as I didn't have it very long before it rolled off a high platform and dropped onto concrete, but that's another story.
So, if I were you I would try the technique and see if it works for you. It's best to take at least a couple of exposures of the same object (if that is applicable for your subject), and then pick the sharpest one. I got pretty good at it after some practice, and sometimes even today I wish I had kept the camera.
I never owned any of the 645versions but I did read in many places that the mirror mechanism is much smoother than the 6x7. I would expect that the shutter would be smoother as well. The bottom line is that most 645 shooters that I'm aware of didn't have nearly the problem that the 6x7 has. But again, I don't have any personal experience with the 645.