Connect animated shape layer to text box for lower third reveal

I designed the animation for a lower third template before adding in the expressions and have hit a block with finishing it out.

I have a bar that builds on the right and moves to the left, revealing two lines of lower third text as it goes. The two lines of text are linked to an optional background box who’s width is determined by the longest of the two text layers.

I want to tie the bar animation to furthest edge of the background box so that the bar reveals the text and optional background layer from their longest point when animating on. I want the bar to hit the same landing position every time. Right now, the bar animates on/from the same point.

The text/background reveal is currently tied to the bar layer. I've tried tying the bar position to the text layers like I did with the background box but can't figure out how to retain the animation/easing and the landing position. 

What would be the best way to finish this out so it works the way I want it to? What expressions should I use? Would a null object function better here?

Current bar starting point:

End result

Thank you!