"String Gummy looks SOO different from the other TBD Cookies."
THAT'S THE POINT.
String Gummy Cookie identifies with being from the future timeline (even after finding out he's from the main Ovenbreak timeline). Especially because that's where he fully rebuilt himself after losing pretty much all of his memories in the Time Pocket he was stuck in. His design also mirrors Director Croissant's a LOT.
The uneven arm designs, tattered cape, even the basic elements of their hair is the same (a strand that goes in front, with the other side primarily going behind the head, alongside long back hair).
The similarities make sense when you consider the fact that Director Croissant is the only person String Gummy had and that she's the one who helped him through (possibly) the worst time of his life (after being saved from the Time Pocket). She helped him rebuild and find himself again and we all know how much she means to him. It only makes sense that he'd incorporate some elements of her into himself.