when you are concerned about battery life and you are not expecting any important calls / text messages, try putting your phone on airplane mode instead of powering it off. The GPS will still work but the battery savings will be huge - especially if you are a bit further from cell towers and your phone is struggling to find a signal. It's the phone/data anntenna's that really suck the battery down (although screen on-time is likely the biggest battery killer).
The advantage of airplane mode vs off is that you can look at a map (saved offline), or read clue (saved offline), or take photo whenever you want.
I didn't read this entire thread, but when I really want to keep a track (especially for adding a trail to the OSM maps), I prefer using my older Garmin handheld with AA batteries - can track for 5-6 hours and if I need to I can just swap out the batteries and I'm good for more hours. It's pretty much all I use it for now, and all my geocaching and letterboxing data file are on the phone.

Map my fitness, Map my run and Map my walk all the same. But I turned the sound off. I didn’t need Ms. Negativity talking to me while trying to find boxes.

I use airplane mode too. Just don’t forget to turn it off Once you’re back in the car