You directly said that grassroots activism, ie Unionization, is akin to grassroots genocide, so I'm glad you're walking that statement back.
The Democrats will not change if you work with them or join them, yes, neither will the Republicans, and third parties will absolutely never gain any traction without significant grassroots change like Unionization.
By voting third party without building up mass grassroots change, you're spoiling your vote and enabling fascism. You can't just vote a third party into office without genuine pressure from the bottom.
Again, read Reform or Revolution. You can't change the Dems, and you can't change the two party system itself, via voting. You understand that the dems can't change, so then why do you think the entire US can change just with electoralism?