If you want to start manga after watching season 1 of anime, start with chapter 73
Tokyo Revengers: Letter from Keisuke Baji
“Tokyo Revengers“ official spin-off : Tokyo 卍 Revengers: Baji Keisuke Kara no Tegami
The story of Keisuke Baji and Chifuyu Matsuno, from their first encounter to their separation. Chifuyu Matsuno, the First Division vice-captain, has not yet accepted the death of Keisuke Baji, the First Division captain, who died in the battle against Valhalla. At that time, a letter from Baji before his death arrives at his hand.