What app do you recommend for designing my own deck of cards?

Here are some resources:

https://educaprimaria.com/portfolio-item/crea-tus-cartas-naipes/

This site comes highly recommended for printing your own decks:

https://www.makeplayingcards.com/

Zazzle makes custom decks with Bicycle quality, and they're not expensive 🤷🏻‍♂️ So, you just need to start practicing your designs 😉😅

https://www.zazzle.es/plantilla+la+del+p%C3%B3ker+bicycle+de+barajas+de+cartas