There is no time to vote for a 3rd party until our system of voting has changed, for example how some jurisdictions have ranked choice voting now. Until that point, voting for a 3rd party only serves to deny a vote to the candidate you would prefer from among the two major parties.
If you think it’s worth sending a message to the Democrats by denying them a vote and making it more likely a Republican gets elected, go ahead I guess, but I will not do that. I will continue to advocate for ranked choice voting in my jurisdiction, but until then I will continue voting for Democrats in all elections.
I would add that only having two parties has not been the direct cause of the current polarization. That has been caused by politicized gerrymandering and a primary system in which a very small number of voters even decide who the candidates are going to be. Becoming active within the party you align most closely with is a way to influence who the candidates are and how they approach issues you care about. Alternatively, you can be like Jill Stein…give up and burn the fucking house down when you disagree with others in your party on a small number of issues