Shanks is Itachi only in the sense that he has a popular fighting style, for Itachi it was speed and finesse coupled with eye powers, for Shanks it's swordsmanship and pure haki. Both are Mary Sues too I guess, the author cannot find it in their hearts to make them look genuinely bad or go through genuine character progression
Itachi was a polarizing character while Shanks is a "safe" character, Oda gave any bad traits Shanks coulda have to his more interesting twin brother Shamrock
N1gga he is child of destiny with genetics of 20 og kings family, his daddy a former HK commander, and adopted by the fucking pirate king himself. He is nothing if not SPECIAL