I was inspired by this article and his use of solar charging on bicycle tours.
He addresses several of your concerns, including the use of battery packs (which is what the Biologic unit also does) to avoid the on off cycles when speed drops/dynamo out put drops below threshold.

Solar battery charging for long tours, by Wayne Estes

https://play.google.com/store/apps/d...solarscreenoff

For direct charging of android devices, the linked App keeps the screen off during stopping and starting charging in order to save battery. Specifically written for the intermittent charging issue that a solar cell OR dynamo hub out put causes. I ran this app a couple of years ago when I tried on the Biologic unit on my commuter. I have not had it on a tour or otherwise since.

Here is a DIY Dynamo to USB project:

https://www.arenddeboer.com/diy-hub-dynamo-usb-charger