The subject of the main clause is neither 私たち nor 政府, but "implicit I" (singular). The subject of a "-tai" sentence is normally "I", and it is normally omitted.
I want to watch the movie.
(私は is almost always omitted)
I want [someone] to watch the movie.
I want him to watch the movie.
I want the government to think more about X.
I want the government to think more about how they spend the tax they take from us.
In all the five examples above, the implicit subject is "I". If you're already confused by the second or the third sentence above, please reread your textbook and review how to use もらう and に together. Also note that the topic of the original sentence is 政府 (because it's explicitly marked with は), but topic and subject are different concepts in Japanese.
税金の使い方を考えてあげたいです means "I want to think how to spend taxes (for someone's sake / on behalf of someone)". It's not an ungrammatical sentence, but only a very arrogant person who believes he is greater than the government itself would say something like this.