Look, my opinion is that the inner workings of the UFP as depicted are rather unimaginative. Frankly, the UFP government you described isn't radically
different from the US system, just a little different. I find it difficult to believe that many other alien societies would agree that the US system is best for them, too. But that's because most of the aliens in Trek aren't very alien, either, being mostly human in both looks and beliefs, with just some funny bits on their foreheads and some exaggerated psychological trait to distinguish them. For me, to be plausible, the aliens of Trek would have to be more alien, and their alien psychologies would have to influence the style of government of the UFP. Not to mention the realities of distance and time in an interstellar society like the UFP; even with warp drive and subspace communications, the UFP shouldn't be treated like the US in size and scope.
Trek literature has the chance to be more different than what can be depicted in TV and films, with time to explain any radically different aspects that are introduced. Science fiction stories should have elements that are both familiar and very unfamiliar. Trek stories in my experience are too timid and use too many familiar elements that don't really enhance the story, but merely make it boring to me.