@Arperrin,

This functionality is not currently supported by the Kasa Application, but we appreciate your feedback. I will send the request off to our engineers to see if Smart Actions can be improved to allow this type of automation.

Right now, I think that the best bet is to create an automation based of sunset and then you can add delays to make adjustments to the lights. While this definitely doesn't give the same effect as having gradual changes over time, it still may be able to accomplish a similar automation. 

Another option would be to look at integrating your Kasa Devices with the SmartThings platform, as SmartThigns has an extremely robust automation platform for its users.