I don't really care much for democracy in the first place, but I've gotta say you guys aren't giving the American system much credit. Yeah, it's really corrupt, inefficient and unrepresentative, but at least the last two parts were basically designed into the system by the Framers. You had Federalists who wanted a fairly modest (by today's standards) government in charge with a very significant number of Anti-Federalists that basically wanted the government to be incapable of much of anything. Most of both wanted to make a system that prevented the government from being able to oppress anyone the way they believed the British did them, and they realized that bad things could come from the majority simply straight up having the power to pass laws screwing over a minority. So they designed a very inefficient system in which passing a law takes quite a lot of effort, with the idea being that only relatively straightforward legislation with broad support from all of society would pass - keep in mind, originally the Senate was appointed (as it still is in other countries eg Canada), so the separation of powers was even more acute back then. Now obviously they didn't succeed in preventing legislation they otherwise would have strongly disagreed with from making it through, but they did succeed in making a very inefficient, messy system in which government programs tend to be far less efficient than their counterparts elsewhere due to various compromises that need to be included to get them created at all.
Anyhow, so far as corruption goes, massive campaign donations and gerrymandering certainly don't help, but it's not as though removing them would somehow make politicians any more honest to their constituents or less corrupt. There is plenty of corruption and support for gigantic banking firms at the expense of the people across Europe, and politicians there actually have the campaign donation restrictions and relatively representative elections. Corruption of politicians can come from a lot more than straight up campaign donations.
Anyway, even assuming politicians were honest, unless they're all unusually knowledgeable they'll be reliant on "experts" when it comes to specific issues like the economy, and those "experts" will probably be from a government agency. What tends to happen, and this is VERY evident in agencies relating to the economy like the Federal Reserve, the Treasury, the FEC, and so on, in such agencies is that people from the private sector enter them for relatively safe, cushy jobs and people from the agency leave for the private sector to make the big bucks. Naturally, both are looking out for each other, so the banks will tend to not do anything that particularly agonizes the government (barring some financial regulation skirting on occasion, which isn't a big enough deal for most to care), and the agency will regulate where possible in ways that favour their friends back in the private sector. Sometimes these regulators don't have absolute control for making things pleasant for their friends though, and they have to get the politicians to bend to their will, yet this isn't hard if the politician isn't an expert in their field (and 99 times out of 100 he won't be), since they can simply flash their "Expert" credentials and convince non-ideological, relatively honest politicians to basically give money straight to their friends. Now of course they don't usually need to do this because the banks are rich and influential enough to basically buy politicians wholesale, but that's what would happen assuming that wasn't possible. Now you could say they could get an unbiased representative of the field, but in some fields that's basically impossible. Just look at economics; if you put a hundred economists in the same room, they'd manage to come up with a hundred completely different ideas for how to improve the economy. The fundamental ideas of economics can radically change in a very short period of time; the basic ideas of 200 years ago would be radically different from those 100 years ago, 50 years ago, or those held today. Yet they don't necessarily move "forward" in the sense that they become any more reliable, since the more mainstream ideas in macro would be closer to the views of the classicists of 200 years ago than those of the more laissez faire economists 100 years ago. If I want to push any agenda, I could find a fair number of economists that would just so happen to support my views. So it isn't really an easy to find an unbiased person in such fields to rely on.