Just for fun, let's make things get tougher over time.

In GameStage.cs, keep track of how long the player's survived with the _elapsedTime variable. Increment it with the Timestep property in OnUpdate.
In GameStage's SpawnBullets coroutine, make the delay between shots get shorter over time:

float delay = Mathf.Max(0.5f - _elapsedTime * 0.01f, 0.05f);

In the constructor in Bullet.cs, increase the max movement speed of created bullets as the elapsed time increases:

float maxSpeed = Mathf.Min(40.0f + (elapsedTime * 1.5f), 400.0f);

_moveSpeed = Mathf.Random(40.0f, maxSpeed);

Bullet bullet = Add(new Bullet(movingDownward, _elapsedTime), 1);