I don’t think Lucky Star and K-ON! are blueprints for CGDCT. They surely influenced the shows coming after them but any popular show in a genre does that. The consensus seems that Azumanga Daioh is the blueprint for CGDCT as before it CGDCT wasn’t a genre and after it CGDCT became a genre. For example, can we say that shows like Gochiusa that came after Lucky Star are based upon it and not on something slightly earlier like Ichigo Mashimaro, and can we say that Lucky Star is truly groundbreaking compared to Ichigo Mashimaro in its content? I would say that shows like Lucky Star and K-ON! are not genre defining but instead popularised the existing genre. If anything, that 2007-2009 era was the golden age of CGDCT, not its inception (2002) or where it finalised its form (maybe around 2005 but harder to say). So my vote is Azumanga Daioh for CGDCT.

Maybe for shows heavily leaning into Iyashikei, Yokohama Kaidashi Kikou is what really set the tone but I’m not sure. I’m going with “leaning” here because SOL, CGDCT, Yuri and Iyashikei are almost always in there together to some extend but some works lean heavier into some aspects than others.