From here:
https://github.com/shiftcommerce/shift-react-components/blob/master/src/scss/10_settings/_z-indexes.scss

Currently at 999

Even though they're overridable, I'd reduce them down to about 10

If there is a reason why they're so high, drop it in a comment on here

For example, in another project, we have these values:

$z-low: 1;
$z-medium: 5;
$z-high: 10;
$z-header: 20;
$z-mobile-menu: 30;
$z-modal: 100;