Do not roll your own entropy scheme
use casino dice on a rough surface
cards are not as random as 6 sided dice
if you want to go down the rabbit hole
others have built tools and Schneier's Solitaire encryption scheme
https://github.com/atoponce/deckware
https://crypto.stackexchange.com/questions/70386/can-a-deck-of-cards-be-used-as-a-practical-source-of-cryptographic-randomness
https://www.itsahlive.com/posts/physical-entropy-from-playing-cards/
others have built tools and Schneier's Solitaire encryption scheme
https://github.com/atoponce/deckware
https://crypto.stackexchange.com/questions/70386/can-a-deck-of-cards-be-used-as-a-practical-source-of-cryptographic-randomness
https://www.itsahlive.com/posts/physical-entropy-from-playing-cards/