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.
I've long thought the only way for leftists to get any real success is we need new ideas injected into leftist philosophy. Neo-marxism or neo-anarchism. There's definitely parts of marxism and anarchism that appeals broadly to the working class today, but there's enough of it as well that they don't want. In a country that has some level of freedom of association, there's no reason far left party membership is as low as it is. You can blame decades of propaganda, and yes that is true. But without injecting new ideas into the far left I really don't think it's realistic to expect a sudden explosion of marxism or anarchism. In my opinion, we need a new leftist doctrine unique to the 21st century.