I chose to stick with a WiFi iPad. But, I very seldom use my iPad for navigation (practically never). I do tether to my iPhone for data, and that usually works fine.
When tethered to my iPhone, the iPad seems to gain location data from the phone - I’ve noticed that when I pull up Maps to look up a location or estimate a drive time, it seems to know where I am with some precision. I don’t know if that would “lag” if I were trying to navigate, or if it would have a greater battery drain on the iPad or iPhone doing so. It would certainly increase possible points of failure.
So if I were wanting to navigate with iPad, I’d certainly get a cellular iPad, even if I just downloaded maps via WiFi and ran it without a cellular plan.