Ultimately, the Democratic Party will only be changed through primaries.

Only so far as they continue to hold and respect them. We're now 0/3 on that front, so... yeah. It's not impossible, but they'll fight you every step of the way and then some. Creating/hijacking a third party and strategically contesting election will probably accomplish your goals faster. Remember that the only reason the GOP was taken over twice was because the party leadership approved of it.

I’m not talking about Presidential primaries. I’m talking about state and local. Make it so the Democratic Party is full of progressives, socialists, social democrats, whatever your favorite brand of left-wing politics is. I’d be dammed careful about communists because that word is a fucking non-starter with the American electorate. Hell socialism is as well but it’s not as poisonous as communism. When the party is full of… I don’t know the proper term because every word I think to use (leftist? Lefties, maybe?) has been co-opted to mean a specific ideology. But make it so the Democratic Party as a whole leans further left, and those people have to be tapped to fill key roles because their presence is so large.

You are absolutely wrong about creating/hacking third party. Ross Perot couldn’t pull it off despite spending about 80% what the major parties spent and capturing nearly 19% of votes in a vastly more friendly media environment. In order to achieve just that lofty level of irrelevance, a third party candidate would have to spend about $350 million and buy Fox News or Facebook. Third party isn’t going to happen. Presidential primary isn’t the place to start.