>Is it democratic that an unelected group of 9 people can unilaterally essentially decide the law of the land in the US?

Are republican (little r) systems inherently authoritarian in your view?

Do you actually fear that SCOTUS is going to start imposing some sort of autocratic control over American government? I'm not. You probably aren't either. There are institutional explanations for this.

I would describe qualified immunity as starkly autocratic, and it was written whole-cloth by the supreme court.

I actually consider myself a staunch (little r) republican.

That said, Machiavelli's Discourses on Livy (a seminal republican text) was quite clear that republican mixed government does involve mixing a bit of democracy with a bit of authoritarianism/autocracy. I do think it is "authoritarian", but I don't think it is necessarily bad.

My question is merely: why can we extend this sort of support to mixed government in our country, but seemingly consider many of the other forms of mixed government in action as pure authoritarian tyranny?

How is the US constitutional system remotely relevant in this discussion?

Because every political system has elements of "authoritarianism"/decisions being made by unelected officials.

For instance, we often criticize Iran as being authoritarian, but compared to many of our allies elsewhere, Iran is actually relatively democratic, even if they have authoritarian "checks" just like we do with the Supreme Court.

I have a hard time understanding what you mean about Iran:

https://en.wikipedia.org/wiki/Supreme_Leader_of_Iran

Seems like a dictatorship to me.