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> But the professor said he'd never do it again because 30 minutes * 30ish students was too much work.

The old German Diplom system (replaced by BSc/MSc about 15 years ago) was based on oral exams. The way this worked there was that there were much fewer exams. For example, in Computer Science I had my first oral exams two years into the degree, and there were only five of them: one in Mathematics, one in each of Theoretical, Practical and Applied, Computer Science, and one for the second subject. Each of these oral exam was 30 minutes but covered the content of four semesters. There was a second round of such exams at the end of the degree and that was it.

To be admitted to an oral exam, one needed to pass the courses that the exam was going to be about. Lecturer were free to set the passing requirements. It was often just something like a coursework threshold of 60%.