Solarpunk

I say all this as a non-American, just to be clear. All mainstream American politics looks right-wing to me.

voting green [...] can have the effect of moving the Overton window and creating conversation

I'd love if this were the case, but the pragmatist in me can't agree. Right now, if Biden doesn't win, the Overton window shifts to the right. Voting Green won't shift the needle, as much as the idealist in me wishes it could. No-ones going to care that 2% more people voted for the Greens this time if Trump wins. Trump's already stated his fascist ideals and the Overton window will shift to the right if he wins. Which means if you care about the Overton window at all, you vote to keep Trump out.

If the Democrats were in a more secure position then I'd say voting for the Greens would be very reasonable. It'd be a way to indicate that you'd like to continue to shift things leftwards. But right now, the best thing you can do if you care about left-wing or green values in the long-term is simply to keep Trump out.

Think about where you want politics to be in 2028, or 2032, or even 2036; do you think Trump or Biden winning is the best path to that goal? Because those are the realistic options right now. And personally, I think gradually shifting the Overton window leftwards is the way to do it, rather than letting the country slip into full-blown fascism and then hoping people will suddenly flip to socialism overnight on the evening before the 2036 election.

they’ve had to push a “but trump!” Narrative to scare anyone else from even running.

There certainly has been some of this. But I also think that's something that's very relevant right now. The time for finding more progressive Democrat candidates was during the primaries. Now, as difficult as it may be, the best thing to do is suck it up, rally behind Biden and fight for your long-term future by doing what you can to keep out the fascists. It'll fucking suck, but not as much as Trump winning will.