Hoping someone changes is not a strategy. Plan around who they’ve shown you they are.

Planning around who somebody might become is planning around a person who does not exist. People do change, occasionally and usually slowly, and none of it can be scheduled around your deadline. The pattern they have already shown you is the only evidence available, and building on a hope leaves you surprised by something entirely predictable.

With the unreliable one in your group, plan as though they will do what they did last time. Build the backup in quietly rather than announcing it. Be glad if they surprise you, and fine if they do not.