Trigger before a "timeline ends"

Hi DanielChodos I'm pleased to announce that I did it! In fact, ChapGPT did all the work. 😄 Let me explain.
I was very surprised when I looked at your .story file. In all possible cases, SlideElapsedTime is only known at the END of the slide. “Elapsed” is a clue, even for me, who doesn't speak English very well. So, anticipating a temporal event based on an unknown final duration is... difficult.

I just spent 3 hours this morning making Chappy sweat over the difficulty of finding the expected duration of the slide. He failed a dozen times. Finally, I suggested he look at the progress bar, since its scrolling speed automatically adapts to the final duration as soon as the slide starts. Indeed, in the debugging console, it has a “max” value of 5000 if the slide duration is 5s, 10000 if the duration is 10s, and so on. Chappy warmly congratulated me on this idea and I was very proud of it. The rest was pretty quick. 
Everything works exactly as you want it to, I think, whatever the length of the slide. Even if you remove the progress bar! Give it a try. The timing (-2s, -5s) of the event is adjustable in the JScode. You can now duplicate this slide and change its duration in the timeline.
Look at the .story file. It's all there: slides 2 and 3.
I'd like to know if this works for you.