Setting Imager crop position dynamically
If the value of imageCrop is a string in the correct format (ie 50% 50%, 0% 0%, or similar), this should do it:

{% set entryImage = entry.pageHeader.first() %}
{% set imageCrop = entryImage.cropPosition %}

{% set transformedImages = craft.imager.transformImage(entryImage, [
    { width: 1680, jpegQuality: 75 },
    { width: 1080, jpegQuality: 75 },
    { width: 680, jpegQuality: 70 },
    { width: 480, jpegQuality: 70 }
  ], { ratio: 21/9, position: imageCrop }) %}