The whole brightness thing is a bit of black magic. I have bulbs have no brightness attribute in the light, yet I can dim them via a scene or the UI, and in automations.
What I have done in a similar way is loop through different colours and noticed that difference light types (bulbs vs LED strips for example) react to the colour looping very differently, and need different delays set because they aren’t designed to have a stream of commands sent to them.
But again, given that you are using a 10 second delay, that also shouldn’t be a problem.
As a hack/trial, you could create (say) 10 scenes at different brightness levels and step through those ?
Also, I presume you’ve defnitively proved (perhaps via the UI) that the bulb does support dimming ?
Does this thread help at all? seems to be talking about exactly the same issue