Tho tbh Shamrock does feel like a Lucci type villain. Like he’s an awakened mythic zoan and he likely has CoC as all the gorosei and Shanks have it.
but there’s absolutely no way with even all that that he can be an extreme diff for Luffy. He ain’t Kaido.
My guess is similar to egghead, SHs fight a minor villain the first half of elbaf, the holy knights. Shams’ failure to make elbaf bend to the WG will force the WG to instead consider simply eradicating the island via Uranus or possibly even Imu himself.
In any case, the Harley will likely be a direct foreshadowing of the threat the SHs will face this arc.
Perhaps Garling’s sword is the hell flame serpent, whereas the UFO is actually a demon created by the forest god(Uranus).