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.
There's a reason right wing governments come to and stay in power. They recognise that while the don't agree with everything centrists or fascists believe, they are a unified force when they work together and can maintain power. Sure, each group would throw the other under the bus (except maybe centrists, they'll just boot lick the winner), but they know that holding power is more important.
The left have ideals, which is where the issue lies. All sides have strong ideals that they are unwilling to grind or compromise on. It's our strength and our weakness. It means that we can be morally consistent but always end up fighting before the power is won.
We need the red and the black to work together. We're amazing at fighting together against something (see Antifa), but terrible at fighting for anything, which is needed in a revolution.
It's up to the individual to find the line of compromise, but we need to do something or face being a perpetually reactive force.