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Sounds good. Maybe take an extra power bank to be safe? Like any setup, worth testing it beforehand to see what battery life you get so you can work it out. Or just wing it and take plenty of spare capacity!
You may know this already but you use less power from your power bank if you run a phone (or anything else for that matter) with it connected so that it's battery is always full, rather than running it down and recharging it later.
Before you spend money on another gadget, have you ruled out using a phone app, such as osmand or bikecomputer?
The advantage is they'll give you much better maps.
A phone takes more battery power than a GPS, but you probably want to keep it charged anyway for other stuff. So how many hours you plan to ride per day, how many days you will be off grid and what your charging plan is dictates how many power banks you would need and if its viable.
Eg a long weekend riding 6 hours a day with no dynamo, probably fine with 1 x 10 ah power bank.