This transition time is a pass-through to devices. Not all devices even support transition time, and I doubt that many support 150 seconds. You might look at setting up two Scenes, and using a Scene Transition to move from one to the other. This would allow the gradual change you're looking for.
Is there any way to set up a scene to go from "whatever everything is" to "Scene=On" over a period of several seconds?
While one can issue individual "dim" commands and such with fade/ramp times--the "Scene" transition time doesn't seem to pass to the devices in the same manner as a Dim command.
Ideas? Forcing the devices to a "starting scene" (e.g., off) could result in undesired "off" changes if they were already on to some other level. I'm just trying to figure out a way to avoid "abrupt" changes.
Yes, use an extra Scene with a capture device. You will need a rule to fire this. First thing is to capture the existing device settings into the extra Scene. Then fire a Scene Transition from that Scene to your target Scene.