Here's my experience. I have Apple's Silicon case. Drive an SUV with center console. The top is flat and has a lid for storage. I have laid phone on top of console and then while repositioning myself in seat laid hand on top of phone. I felt phone sink into lid of console ad the lid bent in. No damage to phone. Did that twice so far. I don't think you will have issues. Also, the phone was face up.
Battery life on the 6 plus is the best on an iPhone and comparable to your S5
I still experice lag at times, it's not a huge issue, who knows maybe future updates will make it go away. Having a note 4 and 6 plus, it's not a huge issue for me.
Between football and medical school my 6plus isn't exactly looked after. I've sat on it many times and dropped it a half dozen times - once down 3 staircases. No bending and no cracks.