I enjoy the loop of building a vehicle, killing some enemies, then acquiring resources to build an even better vehicle, but I think it takes a bit too long to acquire resources. Having to go though another 15 waves to get any upgrades makes it a much less appealing prospect than if, for example, you were rewarded with something at the end of each wave. There's also not enough strategy in playing out the waves to make that engaging on it's own.
I like the clean aesthetic but you need a more varied background for the movement to feel significant, as it is it feels like you're just stuck in place a lot of the time. A more dynamic camera would help with that too.
Overall it has the bones of a really fun game, just needs fleshed out a bit more!