The problem with almost all tablets and smart phones is that their screen is a transmissive display. That is, the light comes from behind and through the screen. What you need to see easily outdoors in bright ambient lighting is a reflective display; that is, one that uses a difference in reflectivity of the image versus the background. The "eInk" type of display is reflective. But they have their limitations. To view them in dim light or darkness, you need an external source of light. (Some are transflective, meaning that they basically are reflective but they are transparent too so a backlight can be built into them for viewing in darkness.) They are slow in changing the display, so they don't work for video. And they usually are monochrome, not color.

I've tried using my car GPS and my Android pohone on my bike in a handlebar holder. They're too hard to see, so I just put them into a small handlebar bag and run a power cord to a small solar panel that I mounted in the handlebar phone holder instead of the navigation device itself. I use the voice output to hear the turn-by-turn instructions, but they're a little hard to hear. I haven't yet tried a Bluetooth earphone, but that may be the solution.

Here's a product to watch. It's a "survival" GPS with a reflective eInk touch-screen display: https://www.meetearl.com/. The engineers seem to have thought of everything that's wrong with existing devices for using them outdoors and solved those problems. The crowd-sourced project is well beyond its goal, awaiting FCC approval before going on the market.