FWIW, I was struggling with the same issue as well.
In the end, I bought a WiFi only iPad with a Bad-Elf Pro GPS. The Bad-Elf does tracking, 10Hz refresh, and can connect to 5 external devices simultaneously, so passengers can follow the flight on their own devices. Plus it's easy to download the log afterwards. The battery life is so long it's a non issue. Start-up is faster than any GPS I've ever owned, including my primary Garmin Aera, and you can place the Bad-Elf on the dash for best GPS reception. (You don't have to. I got perfect reception on the passenger seat.)
The cost difference between the 4G and Wifi only was almost enough to pay for the Bad-Elf.