If you're still looking for a thin case that has the buttons exposed, I highly recommend the Evutec Karbon S (not the SI)
https://www.evutec.com/index.php/product/iphone/iphone6-plus/karbon-s-osprey-for-iphone-6-6s-plus-with-no-logo-cut-out.html. At 0.7mm thin, this is the thinnest case I've used that's still functional. I've used similar cases to the Evutec, but the way the bottoms of those other cases were cut wound up hurting my fingers when the phone was resting on them, like they were cutting off circulation.
I see people mentioning the Veil XT and cases similar to that, but the issue I've had with those cases is that, since the case is so thin and smooth, the cutouts for the buttons were flush with the actual buttons, and it was hard for me to press power or volume buttons without looking at my phone.
The second case I would recommend is the Spigen Liquid Crystal, but this case has the buttons covered (but the feedback of those buttons are amazing).
I hope that helps.