Did voting for Biden stop Trump this last election?
Um, yes? It didn't stop Trump from breathing, but it stopped him from being president this term. Even if it somehow eliminated Trump, there is always the next Trump. There is no point where we can stop fighting to preserve past victories, even as we fight for new ones.
How many times are we supposed to vote Democrat against our own interests
None. Voting Democrat is always in your interests. (At least until something major changes) Voting corrupt Democrats out in primaries is even moreso. It would be nice if we lived in a system that can support more than two parties, but we don't.
Will it always be "neoliberalism or fascism" every election
That's why we fight to take over the Democratic party. Every obstacle to defeating Democrats in primaries has a corresponding obstacle to winning a general with a third party candidate. Winning as a third party is both more difficult, and more risky.