Creating a Mogrt: Using expression control on a keyframe

Hi,

Im fairly new to After Effects but i somehwhat know my way around it and have used expressions before to create dropdown menus in mogrts etc.

I'm creating a mogrt for an editor and i have a lower third animating in, it is controlled by 'position' keyframes. However, the length of the lower third will be adjusted depending on the amount of text in the box. So ideally, i would need the 2nd keyframe to be editable in length (it also needs to be capped at a specific max length so it doesnt affect the safe zone of the comp). I need this 2nd keyframe to hold for a duration of roughly 6 seconds until it animates out.

I think this can be done using the slider control and perhaps a null layer?

I would appreciate if someone could help and provide clear instructions as im fairly new to mogrts and expressions.

My layers below: