This is very well written and well said. If we want change that doesn't come from working within the Democrats, we have to build our own robust party across all 50 states, drawing on strong local support. No one outside of the two parties is currently trying to do this, which speaks volumes about the Green and Libertarian Parties.
Leftists really have no choice other than to vote Democrat. We have to protect people who would be targeted by Republicans. We fundamentally go against left wing values if we don't. I cannot call myself liberal or leftist or wherever in-between if I sit out an election where innocent people will suffer if one of the candidates wins.
I like how you've phrased this -- voting for Biden isn't because you necessarily like his policies or viewpoints, but because you want to protect people from Trump and the Republicans. I'd love if we didn't have to worry about fascists, but we don't have that luxury. As long as they're a hair's breadth away from power, we pick the option that opposes them.