For 17 lbs of grain in 7.75 gal of strike water the maximum possible pre-boil gravity (i.e. 100% conversion efficiency) is about 1.069. No way possible to get a pre-boil gravity of 1.076 with the amount of grain and water you used. Your pre-boil gravity (before sparge/rinse) of 1.063 represents a conversion efficiency of about 90-91%. This isn't terrible conversion efficiency, but it is possible to routinely get 95-100% conversion efficiency with a good crush, a long enough mash, proper mash temp, proper pH, etc.
Your 6.5 gal pre-boil with 7.75 + 0.5 gal of water gives a grain absorption rate of about 0.103 gal/lb, which is pretty typical for an unsqueezed BIAB with a moderate drain time.
Sparging (rinsing) will never raise the pre-boil SG. The sparged wort has a lower SG than the initial wort because it has a lower concentration of sugar. Sparging increases the amount of sugar in your BK, but it increases the amount of water even more, so the pre-boil gravity goes down, but the lauter efficiency goes up (because you got more of the available sugar out of the spent grain.)
As mentioned previously, your biggest issue seems to be much higher boil off than your calculations assumed.
Brew on