You realise your first paragraph made no sense right?
"The main advantage of G5 wasn't blunt immunity"
"Kaido tried every attack in his arsenal and none of them worked"
Why didn't they work? Because of Luffy's blunt immunity. All of Kaido's attacks are garbage outside of his club attacks and Blazing Bagua, ergo, Luffy negated the vast majority of Kaido's moveset. This is not up for debate.
It stated Luffy's physical strength increased. That means nothing in terms of its relation to G4's AP, which increases AP through haki and spring force. So idk what your point is. Fact of the matter is the strongest G5 attack is an imitation of G4.