By fleshing out Zack so much I think makes Cloud a weaker character. We're supposed to have a pretty vague idea of who Zack was and why Cloud idolized him so much and to have the Compilation be like "Oh no Zack was really THIS amazing and also was involved with all this stuff and had more to do with the plot" kind of drifts the focus of the story of Cloud saving the world whilst forming his own identity into something more jumbled and convoluted.

Zack is supposed to be a concept first and a character second. To flesh him out as he did makes Cloud actually less interesting and engaging as a character as a result since almost all of his actions are compared to Zack's which is just bulltrout from a storytelling perspective.