Self Resizing Shapes to Text (TikTok/Insta type text in After Effects)

If you don't mind using a separate text layer for each line, this tutorial might be useful

https://mamoworld.com/tutorials/how-merge-boxes-pins-boxes-ae-tip-5

(using Pins & Boxes)

Hello,

You can't create the white rectangles with different length for each lin with expression. You need to create it manually.
Perhaps you can set you have to Essential graphics, with the source text added as an adjustable property, so that you can at multiple instance of this same comp to a master comp with the ability to control the source text from the outside? That way, different line of text will have different length of width.