The transition option is exactly how you would do that, but I’ve only had success putting that in the scene itself. It does also need to be put into each light within that scene too. This unfortunately means I have two sets of scenes: The regular ones I can toggle through the UI with no transition values and copies of them with transition values that I don’t expose through the UI, as they’re only used by automation.
Yes, but you have to put the transition time in the scene stored on the hub. I had to create a scene with an ios app called “Hue Lights”, and within that app’s scene builder you can define a transition time and it will then work with hass.