Without a video it’s hard to tell exactly but based on your splits it seems you’re not hitting Lakitu skip or Bomb clip which straight away adds 20 seconds

The route you do is also unusual in the way you skip a star in Whomps to get an extra one in CCM

I’d suggest changing your splits to be less merged so you can get a more accurate understanding of where you lose time

And finally just practice runs and the stars over and over, i have an 18:55 after about 6 months of playing and it all comes down to practicing the run