Please don't only start to edit the videos when you fill up the 256GB card. You would regret when you have to recover lost files from it. I've seen too many people lose irreplaceable memories due to card corruption or accidental deletion and it is even painful for me to scan and recover their >256GB cards.
Move them out as early as you can and do regular backups. Videos of your child are much more precious than a whole backup solution.
Move them out as early as you can and do regular backups. Videos of your child are much more precious than a whole backup solution.