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 there is too much focus on what the state will be or not be post-revolution, which is worth pondering, but at this point amounts to navel gazing. We should be focused on revolutionary praxis where I think there is more shared vision.
What mostly rubs anarchist the wrong way is the insistence on democratic centralism. but what does discipline matter when if the Party is not in power? it is ultimately a voluntary association between individuals with no way for any of them to enforce discipline beyond social pressure -- which is pretty standard fare for an anarchist. So there is no reason to bicker over the approach, when everyone is ultimately acting under voluntary association.
And it is fairly sensible to take a vote within a group on group actions, but maintaining also that it's better to let people who really object to participating in something skip the parts they find objectionable without being totally excommunicated. A little flexibility on both sides and there is no reason anarchists and communists can't work together towards revolutionary praxis, since no one really has any authority to wield in the first place.
Perhaps there would be tension as one gets very near to a revolution, but I don't see that being the case for quite a ways, we need to raise class consciousness dramatically before that is even worth considering. There is no reason not to be united around that goal. If we are to be successful, many many more people that the current anarchists and communists will have to be involved, and the consensus on the approach may be much clearer by then.
I also don't see any value to argue over what this country did or that country did in the 20th century. We don't have to re-create what they did, nor likely could we if we tried. both because of being in the imperialist core, because of being under a condition of late capitalism, and because of having at least ostensibly a liberal democracy where elections occur with a peaceful transfer of power (though that tradition might be coming to an end!)
Even arguing over what is currently happening in China or DPRK isn't helpful. Until we are to a point of deciding if we need to choose international allies for support, like real support, getting supplies (weapons), economic sanctions, intelligence, state-level hacking -- whatever true "support" might look like coming from China or from Latin America -- it is mostly just of academic interest whether China is state capitalist or true worker's state of is or whatever else. It's not worth dividing the left over these kinds of squabbles, and any kind of "solidarity" measures (e.g. a resolution to support this state or that group) are not likely to accomplish anything of material benefit for either side, so best to avoid those minefields entirely.
Finally, I sometimes see ML's smug about "well this would have never happened with an ML org practicing DemCen" ok you're right, in fact, it wouldn't have happened at all because that ML org doesn't exist. Why has no one created it? Because most people are not interested in getting involved with an org where they feel people are bossing them around, even if those people are right.
Maybe imagine how there is or could be a pipeline to a more disciplined approach like ML, after people have first been involved in more approachable, softer leftist ideas like anarchism or democratic socialism. Expect that some of them will mature in their views and realize that it is not possible to herd cats to a revolution. But surely they'll be much less likely to reach that conclusion if they feel ridiculed or mocked.
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.