Whatever the cost is for high speed internet service specifically for a car is, it will never be worth it. Like GeForce Now uses like 100mbps for their highest quality stream, data caps and cost for uncapped or like a terabyte a month or something specifically for my car, not worth. A Steam Deck is cheap mini-pc/laptop territory.
If gaming in a car actually mattered, there'd be an option to put a small 720p-1080p display in the middle of the car facing the backseats and it'd just have a Ryzen HX-370 or better tucked somewhere and then people can play pretty much everything a Steam Deck can but with better performance