“The party structures allow a small fraction of the population to control the political outcomes.”
Uh??
Every country, except dictatorships, have political parties. And it’s absolutely not the case that it allows a “small portion” of the population to control the outcome - especially in the US,which has a winner take all system.
You need to have a majority to gain power at the various level of government, and a majority is not a “small portion”.
Ballot access on a party line is key, and a plurality in the primary can get you on the ballot for the general election. Then you need 51% of the maybe 60% of the people who vote.
Unless you are really very wealthy and can buy your way in, no outsider gets anywhere politically without working their way up some faction of the Republican or Democratic organization.