In answer to last question...unlike your Praxis shown, they run flush which is design intent. This is because the bearings are within the 68mm shell and not outside the shell as you show in your picture. The GXP crank is a long spindle crank which can take advantage of an outboard bearing BB which btw promotes the 'purchase' of a spanner wrench on each side to 'thread together' respective cups for a rock solid unified sleeve that won't creak. Praxis was really born out of the inherit design weakness of PF/BB30.

If you pick the crank you want for this bike, the decision relative to which BB maybe best will be clear.

Let us know what you decide for a crankset and I can try and help with your BB decision.

Will tell you what I prefer and why however if interested. Also I am not a fan of GXP with wacky dissimilar bearings ID which is merely a proprietary kluge...actually not much of a fan of Sram or their cranks. Nor am I a fan per se of native BB/PF30 albeit typically BB30 with alloy bores can be tamed more easily compared to PF30 provided Loctite is used. I prefer your aftermarket BB setup aka an outboard bearing design if you don't need the more narrow Q-factor of PF30. Reason is pretty simple. Not only is running bearings more outboard a more solid support of the crank spindle but this allows threading together respective halves of the each side of the BB cups that capture the bearings which ensures the most solid BB aka no creaking. PF30 in its native form is more problematic because respective cups can not be mechanically fastened together. Having the BB flush to the shell does not afford any purchase for a spanner wrench to thread cups together as you show in your pic.

So my preference is R8000 crank with single or dual arm PM with outboard bearing BB taking advantage of the long Shimano spindle originally designed for BSA with outboard bearings. To me Shimano cranks from 105 to DA are the best in the industry and have them on a couple of bikes.