Let's say if I apply for this on Dec 30, and it takes a while to process the application, by the time I receive the EEA residence card it would not be valid, but is it okay if I'm only using it for EUSS? I would not bank on this. I hope I am wrong, but there is a chance that you encounter difficult...

The withdrawal agreement makes provision for those extended family member who are dependent or members of the same household to qualify of the apply for facilitation of residence before 31 Dec 2020. And I understand the start date of the family relationship will be Jan 2021 if they are an unmarried...