With SD cards relatively cheap I long thought about, why delete them at all. Just put them into a box after importing the images/when full. So you still have a physical backup.

Because it's a waste of money, space, effort, and nature as well as an unreliable backup that gives you a false sense of security. As well as a good way to leak information.

But yes, keeping the cards for a while and rotating a large pool can be a good approach. When they're in actual reliable ro-at-rest, checksummed, possibly encrypted, offline backup, the cards can be reused. (Always format in device etc.)