I finally opted for the 8" Galaxy Tab A with a case and mount purchased on Amazon. I am using Gaia, although I also have Polaris Ride Command loaded as well (probably should check out Backcountry Navigator and install it too). I've installed a 256 gig SD card, so map downloads shouldn't be a problem for a good, long while. My initial mounting is to the passenger grab bar via a RAM mount clamp and 6" arm. This puts the tablet in a convenient location, below driving eye level, which is important to me, as I it is more accommodating of my trifocal lenses. I have a secondary RAM mount on the roll cage directly above the windshield, but using this requires that I tilt my head up substantially in order to see the screen through the bottom of my glasses (where my bifocals are). I'll be testing the passenger grab bar location out next week, as we're taking the Talon to the Red River/Taos/Angel Fire, NM area for a few days of trail riding in the mountains.
Here are a few pics of my setup:
Mount without the tablet installed
Mount from above the passenger grab bar (without tablet installed), showing the RAM clamping mechanism and 6" arm.
With the tablet installed. Note that it's in close proximity to the power outlet, which is a definite plus. Note also that I've applied a piece of neoprene padding on one of the switch covers. The bottom corners of the tablet rest on this padding and the rubber passenger grab bar grip, which really enhances the stability of the unit.
I do have concern about what I believe to me the questionable robustness of the mount. It's entirely plastic, and I don't know whether it will stand up over the long haul. Time will tell.
Here are links to the case, mount, and RAM clamp:
I've already downloaded maps for the region, so I should be good to go.
BTW - I opted for the cellphone (AT&T) version of the tablet, as it is edge-to-edge glass rather than the Wifi only version, which is only available with a physical button in the HOME position. I just like the idea of fewer opportunities for dust ingress. It was about $20 more than the Wifi version ($220). But, the cell phone coverage adds another $10 + taxes to my monthly cell bill. Of course, I can always cancel that service, as I'm not on any contract.
I'll report back on performance once I return from my trip. Oh, and thanks to JTW for directing me to the mount and case on Amazon.
Steve