It’s certainly an interesting philosophical problem, finding the balance between the individual and the society. My take on it is this: decisions should be made by the people who those decisions affect. In the case of censorship I agree with you completely- my watching a slasher flick does not give you nightmares. If I were playing devils advocate I might say that it corrodes the national character or something like that- but that to me is a very weak argument.

Despite the ideology that it shouldn’t matter, the lottery of birth is probably the single largest factor on someone’s life trajectory today - changing that is incredibly difficult and would likely require the dissolution of many countries

A reason to allow different people groups to do different things could be uncertainty about what is harmful. Letting the various restrictions and allowances play out can give a better understanding of the consequences of these.

Possibly true, but can we at least agree that a democratic majority deciding to censor something is significantly better than a dictatorship deciding to censor something?

I disagree. The majority is never informed enough to make a good decision on something as nuanced as censorship. At least a dictatorship could, theoretically, be benevolent and act on the advice of experts.

A democracy can act on the advice of experts too via representative democracy with representatives (or appointees of representatives) that rely on experts.

ideally no one should be allowed to censor anything (using state power)

well, depends with whom your views align with more