Flanderized? One-dimensional? In what way was Spike one-dimensional in the latest episode? This is a demonstrably false statement and ignores the entire premise of the episode. Spike starts off as a responsible and attentive servant, yet when given a taste of power, he becomes corrupted and selfish. He literally transforms from selfless to selfish over the course of the episode.
So no, he's hardly one-dimensional and I'd argue he's being written with as much thought he's ever been. Instead of whining about Spike's actions, think about what they mean instead. Why is Spike corrupted when given power? What does this say about his character, his desires, and his relationship with others?
These are questions that are brought up by these actions. To write this all off as lazy is just baffling to me.
Actually, Dragon Quest does have a follow-up: Spike At Your Service. It shows Spike struggling to identify with his own race (following the dragon code is an attempt to fit into his image of what a dragon ought to be) to the point where it becomes clear that his actions are entirely self-serving (ironic considering the title of the episode).
You misunderstand what a plot device is. When the story is literally about how a person is acting, they're not a plot device. Spike would be a plot device if he was greedy and that somehow led to problems for other people and this was all his character was ever used for.
Also, greedy, selfish, and insecure are all legitimate character flaws. In real life, people have more than one flaw. Come to think of it, those 3 characteristics are often found together, so I fail to see how Spike having all of them is somehow a bad thing.