I am trying to figure out how I can use my existing scenes, which gradually dim the lights in the house as it gets later in the evening, to set all the devices in the scene to their set levels instantly.  When I am not home, most of the lights stay off (based on state variables tied to motion detectors).  If I come home at night, I would like the house lights to come on at their appropriate levels straight away.

The inefficient way to do this, would be to have two scenes for each set of light levels.  One would decrease the light level gradually, the other instantly.  I've tried using "fast on", but that just sets all the lights to 100%.  Each scene has around 20 devices.

Am I missing something obvious?

Scenes only support on or off.  You'll need to either use multiple scenes, each with different on levels, or you could use programs to set the on levels.

Create a scene where you want all devices to be when fast on command is issued (all devices should be responders)

Create program that triggers that scene upon a fast on command.