Rabbeinu Tam

Rabbeinu Tam was Rashi’s grandson and one of the greatest leaders of the Tosafists. The Tosafists expanded on the Talmud by asking sharp questions, comparing passages, and resolving contradictions between different parts of the Gemara. Rabbeinu Tam became one of the most respected halakhic voices of medieval Ashkenaz.

For your timeline, he represents the next stage after Rashi. While Rashi made the Talmud easier to understand, Rabbeinu Tam and the Tosafists made Talmud study more analytical, detailed, and debate-based.