Is iPad Pro's GPS supposed to be not as accurate as iPhone's GPS?
WiFi-only iPads do not include a GPS receiver; only the Wi-Fi + Cellular models do. That is why at times you can’t even get a GPS signal at all.

But when there are Wi-Fi signals around or when you have an Internet connection, your iPad can use that to figure out its location.

You don’t need a cellular signal in order to get a GPS lock. I’ve gone on plenty of hikes in the boonies with no cell signal, and ended up with a perfect map. The “assisted” part of "Assisted GPS” only uses cell tower triangulation to acquire a quicker GPS lock.

I don’t see a reason why Apple would disable the GPS radio unless you activated the iPad on a carrier network, so you should be able to use GPS on an iPad + cellular without a data plan.