I don’t think what you want is possible. The best way I can think of to do that would be to put a point control or 2 sliders on the text, and those 2 values would be the start character index/end character index of the highlight part.
If you want to make this graphically visible with guide layers, I think you could put 2 squares or something at the top of the composition and get the indexes based on those 2 layers x-position. Basically, like a volume bar, but to control those values. You can divide the x position by 10 or something like that to make it easier to control(kind of lowering the sensitivity).
You can also add an expression like the following one to those 2 layers to make sure they stay fixed on the y axis.
yVal = 100;//200 for the other one
[value[0],yVal]