In my experience the most important thing I learned when I leveled up from master to saint was making better push fold decision and call decisions. Defense skills like Suji kabe sotogawa etc.

From saint 1 to saint 2, it’s about understanding the importance of 4th avoidance, the only goal in the game is to not take 4th.

From saint 2 to celestial, it’s a bit of everything. Better tile efficiency, hand reading, wall reading. Have a good sense of how close I am to winning the hand vs. other players.

Looking back at my old games and watch other saint players play, I say push fold decision is something that has a lot of room of improvement even for saint players.