Personally don't think Shanks CoC is on another lvl "interms of strength" compared to the other OG Yonko.
Unlike them, the way i see it it's just Shanks utilizes in a form of primary assets and use it as his sources of power.
That's what the definition of specialize in CoC haki.
The Rocks trio all has strong haki, but never untilizes them like Shanks, maybe because they have too much other powerful assets.
He maybe one arm, with no DF but he excel in using CoC and can utilise this in much larger scale that makes him on par with monster like Kaido, WB etc.