Nice shots!
I do want to correct you on the handsome squirrel shot......he was not itching, he was tilting to show you his fine and full belly! Squirrels are always proud of a big full belly!
Sorry about your Sigma lens disappointment.....I've shot for years with the Bigma 50-500 with much satisfaction. My newest HSM OS model is a little trickier to use than my old one, and really doesn't deliver any better IQ, but is still very satisfying. They are big bulky lenses, slow and cumbersome, but that 500mm reach is worth the hassle for me.
The 55-300 you have is a great lens, I have always been impressed with it....and it is bargain priced. Everyone should have one if they don't have other long glass.
Regards!