You're almost there!

Why do you think that a government will oppose a complete system change, but will allow a party change? You're a pol-sci student, you can get this one!

If a government had two socialist parties, would that make it more democratic and less authoritarian? But if both parties had the same goals, what would even be the point of having two of them? Would it actually be more democratic to have two parties, or would that just be a means of enabling the people to feel like things are more democratic, because they get to vote between two parties (but both parties ultimately have the same goals.)

Now imagine say...a capitalist country that does that. That has two parties, but both parties represent the capitalist class, not the people in general. Is that actually democratic? The people get to choose after all! But they only get a choice between two parties that don't actually represent them.

What is a democracy if not a government built around the representation of the people? If the people are feeling represented by their government, does it matter how many parties their are? More parties doesn't mean more democratic. What matters is that those parties represent the people. Even if there is just a single party, as long as the people have proper representation, it is democratic.

The issue is that capitalism has made itself indissoluble. Capitalism is very good at co-opting any threat and integrating it into it. Like how you can buy Leninade in the US (which I hear is just mid). Well, except fascism lol (cf 1945).

Any revolutionary message can be made into a commodity to be bought and sold, reduced to its simple exchange value -- which is something Marx talked about extensively.

Capitalism doesn't need to jail people for speaking up against it. It can defang them completely and integrate their message in the pursuit of profit. Like how Just Stop Oil is being funded by an oil heiress. Was the USSR at a same developmental stage that they could allow people to "suggest" a completely different system?

And the proof this mechanism is working... is that we're having this conversation. Vocal disagreement with capitalism is useless. It does not materially do anything against capitalism. Whenever we turn that disagreement into action and get slightly too close, that's when the arm of the state comes up to ban our parties (authoritarian by your definition), jail us (authoritarian by your definition), close down our media outlets (authoritarian by your definition) and even sometimes team up with fascists to assassinate us (authoritarian by your definition).

But try suggesting in 1790s France that you should have a king again.

And the DPRK isn’t even communist it is a hereditary monarchy masking as “communist” where you cannot suggest a new leader within the party

The DPRK is led by a coalition of three parties; the Social-Democrats, the Chongdu party (religious), and the Workers' Party. I dread to see what they teach you in your polisci degree because this is pretty fundamental stuff about the DPRK, even Wikipedia talks about it, it's not like it's some super obscure factoid.