How about once you hit 20 the difficulty stops, and anything from 20-24 is just shorter time limits, shorter the higher you go.
Starting at 25, the time limit increases, and health and damage go up, and then 26-29 is just shorter time limits with that same health and damage increase. Same type of shortening of time limits as 20-24.
And that repeats, up until there’s a hard break where most people will not be able to push higher.