Hi all, first time posting--so that means I'm still reading the archives looking for previous questions

My son and I are training for the BP MS150 from Houston to Austin next year. It's a 2-day, 170 mile trip.

I'm looking for a good way to keep an iPhone running. It's running a wahoo app that works as a gps and cycle "computer". I'd also like some tunes.
I've got a small tray on the back of my bike where a bag slides in, but I have some room to mount elsewhere. It's a Scatante DX 350, so the cabling is on the top of the top tube.

So, I guess there are these questions:

1. battery pack + USB to phone or just dynamo straight to phone? (is that latter even possible?)
2. size of dynamo? It's an iPhone so it's abound to be doing about 5V DC at very small amps.
3. hub or external friction?

I don't do all-weather riding, so the top-end for durability isn't necessary--save what would be needed for a ride like this.

Thanks in advance for any considerations.