My two cents on how to achieve left unity:
Anarchists: Stop using the word tankie. It pushes your fellow leftists away and pisses them off. Instead make specific criticisms of communist countries. ESPECIALLY stop calling MLs “red fash”. There is already enough misinformation going around about fascism.
Marxist-Leninists: Embrace critical support. Be willing to criticize and accept criticism of nations like China, the Soviet Union, and especially North Korea. Remember that being against US imperialism is not the only thing that makes a country good.
Actually, I think history shows the opposite: actual, material struggle for those gains has a tendency to bring everyone together and make the sorts of concessions to material needs that other revolutionary projects have made seem justified. Just look at the shift in the Bolsheviks over the course of the civil war, or how the CCP during the revolution was a coalition largely filled with and run by peasant anarchists and how their ideology shifted afterwards in the face of the actual material conditions they found themselves in.
That's not to say every single sectarian ideologue would drop the infighting (because there were certainly always holdouts) just that, for example, someone's going to start believing in full purges or suppressions of, say, monarchists and everyone who tacitly supports monarchists (like liberals) when the monarchists have literally been doing pogroms and burying people alive, even if that means you've got to toss them in prisons and you think that prisons are something inherently wrong.