Sentenced to one night of rehabilitation?

you assume jurors don’t have a sense of ethics and justice

I'm not assuming that at all. Jurors have a very specific role, which is to determine whether the evidence against a defendant is sufficient to find them guilty of the charges against them. That does not require a sense of ethics and justice.

It is fixed, albeit imperfectly.

Jurors are instructed to determine whether a defendant is guilty of the charges against them.

To return a verdict of "not guilty" despite knowing that the defendant is guilty, merely because jurors know they can not be prosecuted is still corruption.

Jurors have no idea what "the law" actually is.

When you say "spirit of the law" really what you mean is "the vibe".

The judge's other main role in a trial with jury is to actually run the proceedings of the trial. Order of operations, keeping the two counsels in line, scheduling, etc.

... but in this case everyone is advocating jury nullification so as to avoid punishment, so you don't really need a judge to determine punishment.

Not really. I mean sure some jurors may not like a defendant because of their race, but the court process seeks to mitigate these issues. For example there are 12 jurors and a unanimous verdict is required. The hope being that the majority of jurors will be able to convince a few racist ones to set aside their prejudism.

This isn't really a reason to just throw out the whole process and make trials popularity contests.