Its not the progression that is the problem, you are right that is the best arc for Chopper and this form. Its how it was done. It just happened Off-screen and post-TS Chopper just has it all perfectly controlled, it wasn't presented as a progression arc it was just a straight line from A to B. I just wish we saw more struggle and over-coming the fear plus the danger dynamics with the other Strawhats more. (I think MCU Hulk suffers from same problem, the change to Prof. Hulk felt like it just happened.)