I agree the 800mm is a hopeless lens for insects. Lenses like the 100-400 and 70-200 are not much use for insects either in my experience, though work OK in a pinch. The 100mm macro on the other hand is perfect. For
small birds that are close I agree the 800mm can be problematic and a shorter focal length (e.g. 100-400) usually ends up being a much better choice there. Sadly there is no one-size-fits all.
As for hiking with the 800mm... it certainly has some heft, but I did lug it on a 20km+ day trip in Patagonia once. Nearly killed me and I didn't get any decent shots with it that day, but if nothing else it certainly was good exercise!