Chapter 5 ends with the Roaring, and Sans and the SOUL leaving to Undertale. This is unavoidable, but if you're on the right route then The Machine doesn't become irreparable.
Chapter 6 is Undertale Hard Mode. Afterward you're able to go back and reconstruct Deltarune with a new beginning.
Chapter 7 you play as Susie who has gained her eternity. This chapter never ends, and is Animal Crossing in hometown. This incorporates the alarm clock app's concepts of seasonal dialogue.