Koby's relationship with Luffy not paralleling Garp's with Roger doesn't mean he doesn't parallel Garp at all.
When the discussion is about Roger-Garp's parallel to Luffy-Koby, yes their relationship 100% matters. Otherwise, as I already stated characters can have multiple parallels. Luffy's parallel for "power" for his era isn't Roger, it's his grandfather. Zoro's parallel for "power" for his era isn't Rayleigh, it's Gol D. Roger the strongest swordsman of his era.
And Garp never represented the justice of his era, never, this man's justice is literally "my justice". He was one of the faces of his era, never the sole individual face of his era for the marines. Rather one can argue Sengoku was a bigger face for the Marines than Garp was. Hence the two are constantly grouped together on a consistent basis with no separation in popularity. Especially when it comes to setting the standard for Justice, Sengoku for sure was the face of the Marines over Garp.
Garp, Fujitora, Smoker are the 3 Wild Dogs of three different eras of the Marines. They're constantly shown talking out against their organization's authority and clashing with it. Constantly doing their own thing on the whim that they want to do it. They have heavy influence on Marines around them and under them.
People acting like Smoker wasn't a renowned figure pre-TS that even warlords knew and recognized back even Alabasta and treated with respect to his power is crazy.