Auto-resizing shape

Your expression declares a variable for height that is not used in the size array. Change the last line of the size array to [w + margin, h + margin]

I don't see any that ties the shape layer's position to the text layer's position.  You will also need that. 

The complete solution to creating a shape layer that stays attached to a text layer above it would use thisComp.layer(index - 1) instead of thisComp.layer("Layer Name") so you could add the expression to any shape layer below a text layer. You need an expression on the Rectangle Size, Rectangle Position, and Shape layer Position to keep the Shape Layer Rectangle lined up with the Text layer.

Here's how I would do it:

// Shape Layer/Contents/Rectangle1/Size
src=thisComp.layer(index - 1);
b = src.sourceRectAtTime(); // size of the text box
xPad = 60;// could be a alider
yPad = 50;// could be a slider
x = b.width + xPad;
y = b.height + yPad;

[x, y]

//Shape Layer/Contents/Rectangle 1/Position
src=thisComp.layer(index - 1);
b = src.sourceRectAtTime(); // size of the text box
t = b.top;
l = b.left;
x = b.width/2 + l;
y = b.height/2 + t;

[x, y]

//Shape Layer/Transform/Position
src=thisComp.layer(index - 1).position;

thing 

[w+margin,90] says that the height is fixed to 90 pixels.

Maybe you try

[w+margin,h]

or

[w+margin,h+margin]

Also note that this iExpression could be very useful:

https://mamoworld.com/after-effects-expression/auto-resizing-background-shape

And here is a related tutorial for the even more complex case of the shape being around multiple texts or other layers: