I wouldn’t use a phone as a primary GPS device, I use a Wahoooooo! and have previously used a Garmin. Unless you can USB charge a phone off a dynohub the battery won’t last long enough.
However I have a Quadlock mount on my bike and use a Quadlock case on my iPhone so I can mount my phone on my bars. I can actually charge off a dynohub too. In the past I have abandoned my GPS and used Google Maps to find an obscure address so having the phone bar mounted is handy.
Many phones now are fairly weather proof, Quadlock do a poncho which goes over their cases to give water resistance. We recently went on a beach holiday and used waterproof iPhone cases so we could take our phones swimming and keep sand out, they came from Amazon and worked well. Quadlock do a universal mount which sticks to any phone case and would work with our Amazon cases.
Hope this helps.