You’ve got it right, the vehicles don’t use the Navmesh natively, only character classes do. There are some vehicle AI solutions, but because the vehicles are largely simlike, it’s massive overkill to use them in any situation where there’s going to be more than 10 - 20 running at once, as well as it’s far harder to control the way they move because they are physics based.
Character classes are the only ones to be able to utilize the navmesh natively, so you would either use a character class and define custom movement rules for the vehicles, or create your own more lightweight version but that also means you’d be handling the navigation yourself which unless you’re doing basic A* it could get complicated very quickly. My recommendation would be to put together a character class that you can only send forward/backward and turning commands to, reduce the turn rate, and for the most part it will work like a car. Then spawn 100 of them and test, then 1000, if the game can handle it great! If not, however be wary that character classes are decently expensive, so if you’re going to be going for large scale battles, you may need to write your own.