Setting a scene to transition over 30 seconds works for my other lights, but the flux_led jump from off to on without transition. Maybe the hardware doesn’t readily support this, but HA could simply set the brightness once per second for 30 seconds, or 100 API calls spread over the transition time.
The set_custom_effect doesn’t work at all for single color controllers, and scene transitions don’t call this API anyway.
I wish I could flash the devices OTA with tasmota, as they support the dimming feature well, but I’m not keen to attempt to serial flash the devices.