The only way a 3rd party can be successful under the current system is its they basically replace a major party and become another part of the duopoly. Otherwise they just split the vote with one party and empower the other. We need to promote ranked choice voting or other election reform before we have the luxury of having multiple valid parties. It’s a right, and like other rights, it must be fought for.
One party is fascist, and the other is liberal. Those are the options for president. I’m sorry, I hate it too, but election reform is necessary for more than 2 parties. Not accepting this reality shows either ignorance and naivete, or ignorance and accelerationism. Voting 3rd party will only let fascists win, and fascists are much worse for leftists than liberals. The US becoming a right wing dictatorship without even liberal democracy will not work out for any form of leftism as well as Biden winning reelection.
We need to put election reform directly to the voters in all states that allow direct participation. Otherwise, Democrats are more likely to back any reform, even though they don’t want to. Campaign for Democrats that put election reform center stage, and make it hard for the party to squash the subset of Dems who do want reform. Market it as anti corruption, and focus on that issue above all others. This will draw non leftists who are sick of the 2 party system to your cause if you do it right. You might have to vote for more conservative Dems, but so long as they can be trusted to dedicate themselves to reform, that’ll help real leftist parties become viable. This will take decades of perseverance and getting very involved in local politics, but as the anti abortion crowd showed, decades of hard work pays off.