can I interest you in this link?
https://experienceleague.adobe.com/docs/workfront/using/reporting/reports/text-mode/reference-collec...
you're already 25% of the way there with your first line. You just needed the "DE:tasks:Sub Milestone_Mod=in" line, and then 2 more lines for the status, "tasks:status=CPL" and "tasks:status_Mod=in"
Reporting on collections is super easy because for the most part you can steal all of your text mode from a task report and just put it into a project report and append "tasks:" to the front of every system line, or after the "DE:" for every custom field line. Then for the most part, your task report turns into the validation report for your project report (group by project ID, and run a quick count between the two reports)