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.
It also could be people joining who don't have the exact same views as the people you are thinking of from before. Combined with people who were previously quiet getting more vocal, and people who were more vocal getting more quiet. I know I tend not say anything negative about people who are doing good less efficiently than I think they could because I'd rather they be kinda inefficient than discourage them from acting at all.
I was always extremely skeptical of Bernie, not really trusting him or thinking he or most of the people who supported him so vocally had quite the right idea, but I never said much about it because I didn't want to get in the way of what was ultimately still a good thing. I suspect a lot of people felt the same way, I feel similarly about a lot of things, and behave the same way in response.