Based isayama. He was smart playing the long game with Eren this entire time.
For 100 chapters eren is incompetent, overcome by rage, counter productive, constantly gets his ass kicked, causes harm to those around him, makes the wrong decision, is made to been and idiot, and displayed as truly pitiful with the lowest of the lows.
All of this was set up for the final leg of the story in which all these things make him stronger, and he reaps the benefits by becoming dominant in the story, literally years in the making.
A gary/mary sue is a characters who's always in the right, always wins, never looks bad etc. That's not eren at all.
Eren is just a well written and developed lead, with meaningful tangible growth and iconic persona, for lack of a better term, he's BASED.
I've never been so satisfied with such well done character growth.