And that can imho. not be achieved in a highly partisan two-party system.

That's still tied to the grass-roots level. The party largely can't get candidates for higher office without them coming up through the system in some way. That means that the people at a local level can greatly influence state-level politics, which in turn influences national politics. But the problem you'll run into is that there are a lot of competing interests within a state, and as a single person starts to represent the views of more and more people, they need to reflect the average of those views--or be an exceptionally charismatic leader that can pull people along in their wake. It's not that the party isn't "letting" you play if you don't do things their way, it's that you simply won't have the votes.

Yes, there's a lot of money involved, and it's true that you either need to have a really strong grass-roots funding game, or else you're gonna end up owing rich people and corporations favors. So your issue is that you need to get enough people to give a shit locally, and when you do, they end up playing by your rules. Or, more correctly, the rules of the people you represent.

This is precisely how Trump won, BTW, and how he's come to own the Republican party. That's how populism works. He gave a voice--a hateful voice--to about 1/6 of the American population (about 1/3 of the Republican party), and despite traditional Republican interests being heavily stacked against him, he managed to entirely take over the party.