You might like to try the < group > app.
Only when all group members have the “Duration” option[1], the group device should have it as well (can’t test myself);
Then it can be as simple as:
¹) Unfortunately it’s not a default option for any light. I know Hue and Ikea lights have this option available
Alternative
Dimming over time can also be done by using a Transition for example
Footnotes ↩︎

