David78---when I brought up the usefulness of having a regular old bike computer, the advantages to having one are that given that you will probably plan your days ride beforehand and know the total distance, its an easy way to keep track of progress.
Perfectly good ones can be had for 25 bucks, and while there are wireless and wired ones, (Ive used both but prefer wired) the wired ones are fine (Ive never broken a wire) but more importantly the wired ones will last a super long time on a single small 2032 battery--6 months or a year--maybe not that long on an extended trip--but no matter, a long time so its a simple reliable method that helps with not using your phone all the time or as much, and simply is nice to know at a glance all the time of your distance progress.
think of it as a compliment to gps devices.
re maps, in the past, I've often picked up local or state maps at tourism places to have an overview of a large area. We often go into these places to use facilities or whatever, and sometimes these free maps are worth taking and keeping for the time that you are in the state and then discarding. Sure, some of these maps arent of much help to a touring cyclist, but sometimes they are.
have you read about dyno hubs to keep a battery bank charged? As someone else mentioned, there are always technical hiccups with these things, so researching all the details is needed, but folks do use them successfully so there has to be dependable setups out there.