I'm not the person you asked and they surely have a better answer, but I thought I'd throw some things out there:

A lot of what are understandably called "one party states" are not technically one party. The DPRK has like three other parties and I'm pretty sure there are countless parties all over the PRC. It's still reasonable to refer to these countries as one party states because they have some kind of constitutional provision preventing any other party from taking power at the highest levels, but they still use multiple parties as a means of representing diverse interests.

Our comrade VI evidently knows way more about Cuba than I, but something I happen to know is that, when you run for office in Cuba, you are not the candidate of any party, you are effectively independent. I think that they conceptualize what a party is in a very different way. In America, the political parties are literally private entities, with all the legal ramifications that entails, and are effectively companies pushing brands in order to get money from donors via held seats (that's a crude generalization, but I think it works well enough). In the "one party states" I know of, the "one party" is considered to be part of the governing apparatus itself, rather than something that exists outside it seeking to influence it. It's all a conjoined project that way.

I personally think that, assuming there is actual democracy in terms of the government needing to enact the popular will, a one party state is probably a more coherent way of having society united in its various projects, even if the proverbial ship needs to change course now and then for whatever reason. That's just my feeling though, and it's mainly informed by the overwhelming sense one gets if they follow American elections that they are engineered at every level to be anti-democratic.

Thanks for being open to discussion, I appreciate it. I'll start talking about the reality of multi-party systems and liberal democracy.

Generally, multi-party systems aren't democratic if we adhere to the definition that "the power of legislation is in the hands of the people", which I think would be a good premise for a parliamentary multi-party system. Ideally, you'd choose a platform in elections, which has a given program, or even create your own platform if you don't feel represented enough. Then, this platform supports its program in a Congress, and votes through representatives to pass legislation according to its program. Sounds good, but let's examine whether the policy that people want to enact is actually passed, and whether policy that people don't want to enact is passed.

We can start with the case of the US. The vast majority of Americans support an extended universal healthcare system of some sort. The technical details are a bit hazy, but the reality is that most people would support such a system as poll after poll shows. Yet, the years pass, and there's basically no progress in this direction, how is this democratic? How come if a majority of people support this, it's not pushed forwards and legislated? It's the same with abortion rights, a vast majority of Americans believe in legal abortion rights for women, yet no legislation is passed in that regard and many states actually go backwards. A majority would support increased taxes on the extra-wealthy and on big companies. Study after study show that public opinion is one of the worst predictors for policy, i.e., there's barely any correlation at all between polls on policy, and actual passing of policy. Can we say that there's an actual democracy in the US, when the interests of the people don't correlate with legislation?

I'll talk about Europe now, since I'm Spanish and it's a closer example to me. Recently we assisted to the outrageous example of Macron unilaterally skipping Congress to increase the retirement age against the desires of basically the entirety of France. Huge protests broke out, he was vilified in social media, and all polls showed that this was an extremely unpopular decision. Yet it passed. The same happened all over the EU during the 2010 Euro crisis. Austerity policy was enforced by the authorities everywhere: lowering expenditure on healthcare, education and public retirement pensions, reducing investment in infrastructure, increasing taxes such as VAT... Again, this was extremely unpopular and against the desires of most people. It's been a decade and a half since then, and these austerity policies are still in place. VAT is still higher than it was, expenditure in healthcare and education hasn't increased to the levels prior to the crisis... Yet another example of blatant anti-democracy. If the policy isn't carried out with the will of the people, the system isn't democratic.

I could go on giving examples of failed cases of policy in multi-party systems, but now I'll do the opposite and bring examples of multi-party systems that actually applied popular policy.

Salvador Allende was a Chilean leftist politician in the previous century, who was elected by a majority of citizens to carry out nationalizations of the mining industry (the heart of the economy of the country at the time), and to improve the welfare state. His term didn't last very long at all, because when popular policy started being actually enacted in a democratic fashion, a fascist coup murdered him and replaced him with a fascist dictator.

In the Spanish Second Republic, a similar thing happened. In the 30s, a very progressive leftist government was elected, and promised to carry out land reform, i.e. expropriation from big landowners and redistribution of land to the farmers in a country which was primarily agrarian. It suffered the same fate: a fascist coup, a bloody civil war, and almost 40 years of fascism.

In Iran, under the administration of Mosaddegh, a leftist secular politician who wanted to make sure that the Iranian oil was profiting the majority of Iranians instead of the Shah and a few British companies, nationalised the oil industry. This was met with economical blockade, with paid actors pretending to be communists destroying private property to agitate people, and fake protests organized by the mafia funded by the MI6 and the CIA ousting him from the government.

Now let's go to a period in which actually progressive policy was passed in Europe in a popular and democratic fashion: the post-WW2 period. Under the looming threat of a socialist revolution, and the high level of labour organization through unions, the governments of Europe were successfully pressured into passing meaningful legislation on the limit of working hours per week, on progressive tax systems, on welfare state (healthcare, education and pensions)...

So it seems to me, that the only way to make governments pass actually progressive and democratic policy that most people agree with, is through the organization of workers and the threat of a communist revolution. That, if people just vote socialists into power without organizing labor, they suffer coups, that if they vote social-democrats they get austerity and antidemocratic policy. What percentage of Europe agreed to increase the military budget after the start of the war in Ukraine? I'm not trying to argue whether that's a good or a bad policy, I'm just saying all polls showed it was an originally unpopular decision, yet it was carried out.

If the only way to enforce governments to enact popular, progressive and democratic policy, is through the organization of labor, then why would I want multi-party systems instead of a system of representation of workers in a single, unified, democratic structure?

I know it's a long answer, but I appreciate it if you made it to the end.