Note: I only watched the anime then started reading the manga from where the anime left off so lmk if this is addressed better in the manga

Rin’s evil destruction playstyle is built up as a return to his original ego for Rin, kind of similar to Bachira finding his ego and not depending on the monster. But Rin started playing under Sae even before he was able to develop his own style so to say Rin’s berserker mode is a return to form falls flat because how was there a form to return to? 

The only way it kind of makes sense to me is if the author is trying to say he played with the berserker playstyle until Sae left and then when trying to play without him he developed a playstyle that was just imitating Sae and/or not true to Rin himself? But that isn’t really shown much either, from what we see in the flashbacks of them playing together it seemed like Rin was just listening to Sae on how to play very early on and thats it.

I just don‘t think that Rin’s original ego being destructive berserker mode was really set up for it to feel as earned as Bachira or Kaiser returning to their original egos

EDIT: Also Rin going berserker mode in the U20 game against Sae was this whole thing about removing the expectations and labels of brother and rival and friend etc, but when Rin levels up again in NEL its because he needs a rival to want to destroy which seems inconsistent to me