First congrats on your new camera and NO, I don't think you will be disappointed with it, it might not be the best camera in the world but I am sure the 7D II will be excellent.
I do have a feeling the next 6D will be close to the D750 and would probably be the camera you would have chosen if it was available now.
But then what the heck do I know
It came! And my husband hid it for 3 weeks:-(((
I also have that feeling that 6d mk ii will be what I would like, but I know even less than you

I just hope it doesn't come out two months from now or I will be a bit dissappinted. I did concider waiting to see if they announce it in a couple of months before buying anything, but my Husband told me I am insane and the place where I work just paid me a bonus and told me to go buy a camera

I think 6d would have handled gymnastics just fine, it's not very demanding on autofocus, I think. I didn't had lots of out of focus shots in fact I got plenty of great shots last season with 60d, I just wasn't happy about the noise at times especially on crops. I might just rent one for a weekend when girls are going to nationals in June and compare.
Shooting dogs is a whole diff story, so I am sure 7dii will be great for that. I can't wait to test it out!!
I am hoping a couple years down the road to be able to add 6dmkii as a second body.
Yuliya