Ok, lemme rephrase - while I also acknowledge the PoS system needs to be setup in a way that enables quick and efficient checkout, my head was more in the space where many regular restaurants and bars have gone.
With whatever service they could go with, it'd be connected to your membership and any dining pass plans you have plus any forms of payment (or only forms of payment if you don't have either of the other things).
So whenever you're ready to order items (assuming there'd be a pickup window for this instead of buffet selection), you just pick what you want and payment is automatically handled without holding up the line.