I'm throwing an idea out there and haven't fully tested it yet.

there is a BuyerJourney input at the cart level I believe and have these values:

CART_INTERACTION Buyer is interacting with the cart.

CHECKOUT_COMPLETION Buyer is completing the checkout.

CHECKOUT_INTERACTION Buyer is interacting with the checkout.

I bet you could do a logic gate for if buyerJouneyStep is equal to CHECKOUT_INTERACTION, and then run your validation logic there.

https://shopify.dev/docs/api/functions/reference/cart-checkout-validation/graphql/common-objects/buyerjourneystep