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.
We criticise and discuss those all the time
Take Espresso Stalinists page which is a great place for leaning and where you can read detailed critiques of actually existing socialist countries
https://espressostalinist.com/marxism-leninism-versus-revisionism/
However the "criticism" you refer to is basically other leftists repeating imperialist lies as truth like "million uyghurs in camps wah" or the "millions in gulags" (if we were back in Stalins day)
Now we know that the US imprisoned more people per capita than the Soviets did (at the height of the gulag period on the eve of ww2!!!)
thats just another detail down the memory hole - the stick was used to beat MLs and their support for the USSR back then and can be safely discarded now the Soviet Union has collapsed