Smart path to recovery after a drive failure?

Aloha! Apologies in advance, as I feel like there's a simple solution here that I am just not seeing.

I have a DS223J with two mirrored 12TB drives, plugged into my router's only ethernet port. - One of the drives has died.

I just purchased a second DS223J and three new drives. I am trying to understand the safest way to get all the data on my remaining drive onto all three of the new drives. I know I can do this by dragging and dropping files on my computer, but from lurking here I believe there are much more efficient ways of duplicating data from one NAS to the other.

Do I swap the dead drive before or after I try and copy all the data to the new unit?