I'm always weirded out by the sentiment that a vote for not Harris is a vote for Trump.
I totally get the logic and the math. I think anyone voting for Stein is making a major mistake (and anyone voting for Trump is making an even bigger mistake). It just doesn't feel like a winning argument. I've voted for third parties in the past and at no point did I feel like I was voting for the Republican candidate. (Perhaps you can argue that my past self was naive.)
There are so many good reasons to vote for Harris and the Democratic party. I feel like time is better spent raising those up rather then just pushing people down.
Again voting for Stein is an absolute mistake. I think even if you supported her in the past her recent statements make it very clear she isn't trying to actually win or make progress towards changing the overall conversation.
Also just to get ahead of it, I assume the majority of Stein supporters support her based on her views on Gaza. I admit that the Biden administration hasn't done a great job in this area. Similarly at the moment we don't see much different from Harris.
However there is a zero percent chance of the situation getting better under a Trump presidency. It's a definite continued genocide if Trump wins. There will be no discussion or debate, just more killings.
The only chance for improvement is Harris. You absolutely should criticize the Biden and future Harris administrations. You absolutely should keep the pressure on. But when you vote, you vote for the option that actually has a chance at improving things. You don't have to be 100% happy with your vote, but you can vote while still pushing for change.