Kaido's kid started LARPing as the guy who was going to kick his ass until an old woman pretending to be his son tricked Oden into getting attacked head on.
Shanks isn't scared of Blackbeard, he recognizes him as a healthy threat.
Kaido's PTSD is clear when the Scabbards are attacking him: he wanted Oden to kill him, and it eats him up that his glorious death was ruined. Seeing the Scabbards come back to life to haunt him has to honestly fuck with Kaido's head a ton.