Combine two light entitys into one (DMX / Modbus) problem with slow response

I have set up the following entities:

Modbus light
This is a relay in a PLC that switches the power to the light on and off. This relay is also switched by a physical switch.

DMX CW/WW Fader
Can change the color temperature and brightness of this light.
Cannot be physically switched/changed.

Now I need an entity for the front end. Since I physically switch the power via a relay, the lamp turns on and off quickly, but the DMX dimmer must always remain set to the last value and not be dimmed to 0% when switched off.

How do I do that? The entity should still always have the correct status of the light.

What I have already tried:
light group
Virtual Components and rules
helper light

Can anyone point me in the right direction?

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