I'm using an old Motorola Moto G Android phone (4.5' display), it was my wife's until she needed an upgrade. I factory reset the phone and configured to be just my "flying phone" with Skydemon, TAF/METAR apps, Windy, etc and a custom home screen. It stays in my flight bag, as does a RAVPower 22000mAh external power bank. I charge both overnight before going out and load in the skydemon route (which had been saved to the cloud after being created on a desktop version of skydemon) in the morning while doing last minute pre-flight and printing.
For flying, I mount the Moto G on a RAM suction cup just to my left on the windshield near where it meets the dashboard, and connect it to the power bank that sits in the door pocket. It's not as intrusive as a tablet, and I haven't needed the entire real-estate of a tablet while flying. I ALWAYS print out paper copies of the plog, plates and circuit diagrams for my kneeboard, and I mark up my half-million map (which I still do occasionally refer to for situational awareness) with the route. I always type and print my own route brief anyway (legs, frequencies, circuit altitudes, noise abatement notes, ppr numbers, etc) for my kneeboard. I've not had the device fail in flight, but I'm not taking chances.
I've had instructors and other pilots see this minimal cockpit set up and like it more than their tablet configuration.
I do have a tablet (similarly, a repurposed Google Nexus 9 configured entirely for flying) that I also charge and bring along, but I don't use it in flight: it's where I lookup information, read docs, adjust skydemon routes, etc, always while on the ground in the club or at an airfield cafe, etc. I bought a yoke RAM mount for it, but still after half a year of flying I've yet to have to set it up, simply no need. But it is a backup.
Never had a problem with sun and heat. But I know from experience that this can be very device specific. One of the old Google Nexus phones (Nexus 5 or 5X?) was notoriously known that when used as in-car navigation devices, they drained power (and heated) faster than they could be charged, due to intensive GPS and and computing load.
So I second the sentiment here, you don't need top notch hardware for in-flight. Don't waste money or put high-end devices at risk. Keep the high-end device for use on ground and elsewhere that performance and screen space is needed.