Problem with Ibushi is that his character hasn't changed, ever. Pure whitemeat babyface but he never even has periods of self-doubt. He's won the last three A-Blocks now? And of course won the B Block in 2018 too. They've turned him into Mr G1
Reached the last 4 G1 finals, won the last 2 G1's in all.
I think the big problem with him is as you say he has no character arc, no self doubt, but also, no real organic rivals anymore now that Omega has gone. They kinda have him and Okada, but that feuds has (relatively speaking) had so many matches in the space of 2 years after nothing for years that it feels forced.
On the character, even Tana, the most Babyface of all babyfaces, had periods of adversity, particularly as he got older and the question creeps in of if his time had passed or can he still be The Ace.
I still very much think Zack should have won Block A. The story fell in their lap for him, and it would establish a new, young, foreign heel main eventers, who wrestles a style that will hardly ever injure him or his rivals, is a good clash against other competitors, had great matches with pretty much everyone, who has committed to Japan (unlike Ospreay and White who I wouldn't be shocked if they moved, although maybe not now with WWE's change in recruitment)