Syllabus is maybe 40% done (polity decent, modern started, economy weak, environment not started). Some friends already started a full prelims test series. If I start now I will just be guessing.

Should I wait until a first full pass of the core books?

I'm in the same boat and I'm only doing topic tests after I finish that topic, not a full 100-q mock every week. A full mock too early just scared me without teaching much.

A sectional or topic test is fine. A timed full mock is more useful when you have at least one pass of polity, modern, eco basics and some CA. Otherwise the analysis takes longer than the paper.

makes sense. I'll hold the full mocks and just do polity/topic quizzes for now.