I'll refrain from commenting on your loss of faith in the dealership, that's between you & them, but, that out of the way, I'll agree with the recommendation.
My "in-car-navigation" experience starts with Rand McNally (paper maps) in the 1980's and progresses through stand alone GPS systems to integrated NAV systems and for the last decade or so Google Maps, now via wireless Android Auto. Updated maps, or a lack thereof, have been a constant bugbear through most of this journey, up until I started using Google Maps, I no longer have the struggle that you are currently facing.
Google maps are updated far more frequently than any integrated Nav system will ever be, and without user intervention, mobile data usage is minimal, and whilst nice to have, not really necessary - you can use "downloaded" maps and keep those up to date when you have a WiFi connection. I've used Google Maps in the mountains along the California Coast in areas which have no cell service, it does require "pre-planning", but it's significantly less of a headache that having to purchase the map updates (that's right, they aren't free) and load them.
There are also added benefits, like traffic updates, road hazard updates and very convenient, you can sit at a computer, "pre-plan" your trip, and then upload it to the phone - try it...