I don't own a handheld GPS. My smartphone does Just Fine Thanks. I produce my own maps for it - which means that my maps show what I want them to show (and sometimes I'll render a single-purpose map for a specific trip).
I carry a 15 Ah power pack. With reasonably careful power management, the external battery lasts longer than the amount of food I'm willing to carry on a section. I have a full-on ruggedized pack, which means that it's the form factor of a brick, but it's also survived a dunking in a river.
Delayed signal acquisition when out of cell coverage can be an issue - but it tends to be an issue only if I've not run the GPS for about 24 hours. As long as I acquire signal when I'm still in coverage, it will have a current ephemeris downloaded and be good to go.
For what it's worth, the entire track of
https://hiking.waymarkedtrails.org/#route?id=4286650 was recorded on my smartphone, on three charges of the external pack. And I'm slow. I planned 8-10 mile days on that trip, and found by the end of it that 12-15 was more comfortable. Long Lake to Piseco (plus a side trip to Wakely Mountain) was a six day food carry.