I may be wrong but I believe that end: top bottom means to end the animation when the top of the animated element reaches the bottom of the viewport. So end: top bottom-20px would end the animation when the animated element is 20px below the bottom of the viewport.
After some fiddling, the solution I came up with was to conditionally render the children. The scroll requires a ref to the scroll container so the init function is placed inside useEffect to guarantee the ref is defined. And because this component is at the "top level" we don't need to pass location to the useEffect callback. This seems to work but any improvements or suggestions are welcome 🤷♂️