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 think people think that the alternative to "left unity" is actively promoting infighting which I don't think is the case. Left unity is a pipe dream (imo) but that is just because, as a Marxist, I see Marxism as a science that is emprically sound and correct. Still, I recognise that I, myself, am a product of the lib --> radlib --> ML pipeline so I think we should promote a culture that isn't hostile to people that aren't Marxists, but also doesn't promote a liberal idea of ideology where anarchists and MLs differing only over "aesthetics" or "authoritarianism" or whatever when it is actually modes of thought with different class characters.