When one should join Mains test series for CSE after prelim or before?

Do you not believe that commencing your preparation in full force right away is the best course of action? There is no need to wait for the prelims to end and then for the prelims results, both of which will waste your time. Like how we left mains preparation two to three months before the prelims, and how we focused all of our preparation—including optional material—on the mains syllabus before that—leaving test-series at that point can hurt your performance.
Thus, deciding whether to start the mains test series for CSE—after prelims or before—is just a complicated decision. Just remember that if you're preparing for the UPSC CSE mains, your preparation isn't complete until you've taken a mains test series.
In-depth research and surveying led me to the conclusion that EDEN IAS, one of the best IAS coaching institutes in Delhi, was my best option. For this reason, I enrolled in their STEPS program, where I learned the fundamentals of writing mains answers that covered the entire GS syllabus.
Additionally, the mentorship sessions serve as the UPSC students' power bank throughout their preparation, helping them to clear up any issues they may have and directing them towards improving their marks.