How to animate style text from start to end with expression?

Hi everybody!

I'm looking for using an expression to change the style of my font but gradually like how we can do with position, opacity etc... on the text animations provides by After Effects.

I'd like to have different font styles for a sentence. And use a Slider to change the style of the characters of this sentence gradually.

I created a text layer with multiple letters. Each letter got its own style.

Then, I create another text layer with my sentence. I created a slider effect on this text layer and put this expression in the source text panel:

thisComp.layer("TextStyle").text.sourceText.getStyleAt(effect("Typo")("Slider"));

When I move my slider, all the sentence change its style! Perfect. But now, I'd like to have this change gradually, maybe on 2-3 letters to have something smoother etc... But what I have to add to this expression to manage it correctly???

Thank You in advance for your science!!!

PS: Im' french 😉