Fading out a smart lamp

I want a bedroom lamp to remain on dimly for a while and then to fade to off over a few minutes, This seems like a fundamental thing to do with home automation.

The preferred way to do this would be to make a script setting a variable for the intensity and then going in a loop, decrementing the variable and setting the intensity accordingly. Unfortunately I can’t see a way for HomeKit to set intensity according to a variable.

I’ve made a crude workaround by using Shortcuts. It initially sets the lamp at 15% intensity for “OnTime”minutes and then there are 15 steps, each one setting the intensity 1% lower and pausing for “Delay” seconds before moving on. It’s a very crude workaround, but it does work and I can quickly tweak “OnTime” or “Delay” to adjust the settings.

Some automations can be converted to shortcuts, but only using a tiny subset of the commands possible within the shortcut app.

It’s a bit of a nuisance having to trigger it from my device. I have an Omvis 5 button smart switch and it would be great to use that to trigger the shortcut, but I can’t see a way to do it.

I also considered using a spare smart switch as a sort of intermediate step. Put the switch on, then when that event happens, the shortcut is triggered. Unfortunately I can’t see a way to trigger a shortcut from an event such as a lamp turning on, even though an automation can be triggered in that way.

Has anybody got any clever ideas about how to make an Omvis button trigger a complex shortcut, or to trigger a complex shortcut from an automation? There are a few other automated features in my home which ideally would work best via a reasonably complex script, but the script needs to be triggered easily and by anybody.