Sigma or Tamron 150-600mm Lens
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I have been using multiple cameras with attached lenses for about six years. It works very well when using side bags. I can pull them out very quickly. So what you write has merit for us that choose to do it that way.
If you considered one of the Sigma or Tamron 150-600mm telephoto lenses you might be a little surprised at how easy they are to use, once they are handled properly. At first my hands and wrists were a little sore when I started using it for a couple of hours. Then I learned to cradle the lens and camera until I was ready to take some handheld pictures. Now it's easy and I never get sore. Some people also use monopods or tripods so the weight issue isn't as great on their hands.
I think when I eventually jump to a 24 megapixel camera from a 18 megapixel camera, it won't be that much of a notice (in terms of resolution for cropping to get closer). But when I jumped from my Canon 55-250mm IS lens to the Tamron 150-600mm lens it was a huge difference at the long end. I'm still planning to upgrade my 18 megapixel cameras, but I have been slow doing it. It would be neat having 24 megapixels and better high ISO performance etc., but (in my opinion) the huge telephoto lens is what can propel us quickly into the big leagues (much more than going from 18 to 24 megapixels for wildlife pictures).
Here's another photo from Friday, December 30th. I was lucky to get these because we have had lately so much clouds, fog, rain and snow in the Portland, Oregon area.
This is a female Hooded Merganser. I cropped it slightly for composition and to get a little closer. I could crop it a lot more and still retain the details. But I liked the water surface too. It's one of my favorites with the reflection. At this time of year the sun is very low in the horizon, so the light shines on the side of the bird instead from above (like in the summer).

Happy New Year again.