A clear ending knows which question the work came to answer.

Work drifts. What began as one question grows extra aims along the way, and by the end it can be hard to say what you were solving, which is why so many endings feel arbitrary rather than complete. An ending lands when it closes the thing that opened it, even if plenty of interesting material had to be left out.

Write the original question in one sentence and keep it visible. Near the end, ask whether that question has been answered rather than whether everything has been included. Let the leftovers become the next piece.