Pretty good for a first go with a new camera. Yes, I can see the f/stop and shutter speed issues on a couple, but still...there's some decent stuff going on as well. I looked at a couple of my favorites in Lightroom, and made a couple of tweaks, just to see how things looked. As far as the "gloomy" weather...I love a foggy background in the winter time! As far as the tips you asked for? My tip would be to avoid shooting action like this at f/2 with a 50mm lens, and instead, stop down to f/3.5 or so...it gives a shallow DOF and a blurred backdrop, but also gives a little bit more depth of field at these 10 to 20 foot type distances, where the 50mm lens is so, so good. You did a good job keeping the shutter speeds FAST!, to stop motion, and used some of the 6D's good, higher-ISO capability. These look a slight bit under-exposed to me, however.
I took the liberty of cropping this fine shot of yours, eliminating some space on the left side, and giving the dog "more room to move into", which I think creates a better balance and a much stronger,
excellent picture. The out of focus foreground rock sort of "points" to the dog; the shutter timing you show is fantastic! and the animal's overall positioning is excellent, and the low camera angle...all, very nice!
I gave this shot a little bit of a vignette and an exposure/color tweak. I think the balance of animal.action,and background, and low camera angle...all things considered, make this a very good pet photo.