Sorry. I wasn't trying to add to your anxiety, just relating my experience. I've shot a lot with a series 3 and lenses up to 600mm, but I wasn't as successful at first as I am now. It takes practice to shoot the long glass well.
I think Moose's shooting techniques are great, especially for Moose. They may or may not work well for you. I don't keep my gimbal all loose, but instead keep a slight bit of tension on all the movements. I just works better for me. I don't use the "roll the finger" technique on the shutter either, but then I've always been pretty good at gently pressing the shutter button. I think you need to experiment to find out what works the best for you. I also know that's really difficult with this trip coming right up.
Others here are giving you really good advice in troubleshooting the problem. My advice is to follow their lead, eliminating only one variable at a time. But most of all, I advise that you don't fret about this so much that you don't enjoy Yellowstone. It's a magical place in winter. My guess is that you'll get good shots, and some bad ones. That's how it usually works for me, anyway.