If the parent due date could automatically be set for the soonest subitem’s date (like we did in the above example, March 9), but then automatically switch to the next subitem due date as items are marked as “completed”, like in this example (all “March 9” subitems are marked as completed, so the parent due date shifts to March 16):
We have an app called Rollup Subitems (see https://monday.com/marketplace/10000124) that almost does what you describe. It can copy the first (or last) date from a set of subitems to the parent item (as real date) and it can filter out subitems based on criteria.
The filtering is currently a defined list of terms against a dropdown column in the subitems.
The Rollup Subitem app can filter on status value and can put the earliest date of those subitems that have the status XYZ into a parent item date column. Is that what you are trying to achieve?