I think the aperture of f/8 was simply for a sharper image. My Sigma lens is slightly soft even at it’s minimum aperture of f/5.6!
As to overexposing, I am of mixed opinions about it. As Paul Burgess said, the tree trunk is already distracting, and if it was brighter, it would be all that much more distracting! Of course, if I was going to do it over, the tree trunk wouldn’t be there, so I could follow your suggestion too.:)
I’m afraid the “central composition” that you like so much, was more of an accident. i.e. I didn’t sit there and think, “Wow, what this picture really needs is for the puppy to be smack dab in the middle”. I have the bad habit of tending to center stuff in the frame, I think mainly because I always use the center focus point for focusing, as it’s the only cross-type point on my camera!