FWIW, looking back at the template today, I realized it can also be written like this:
action:
- service: scene.turn_on
target:
entity_id: >
scene.front_porch_{{ 'bright' if trigger.to_state.state == 'on' else
['savanna_sunset', 'meriete', 'moonlight', 'starlight',
'forest_adventure', 'magneto', 'arctic_aurora', 'memento', 'motown'] | random }}