The best practice is to generate a different bitcoin address for each visitor in the restaurant, which would result in a different QR code each time. Firstly, reusing the same address twice is discouraged. And second, trying to distinguish users by amount they sent or by correlating the time they do it is a very unstable strategy. What if the payment gets delayed? What if you have more than one checkout point/waiter with the smartphone? Generating a separate address for each check solves all problems.

If it was not for a restaurant, but, let's say, a public display with the address to send donations, and you wanted to hide the total, you could change the address each time you receive a payment there.

And finally, what you propose can fully be done only in complete hell-bent on privacy blockchains like Monero. In Monero, you can display your address publicly, a sender can see their transaction confirmed, but they can never know how much XMR went into the same address from other senders, and, by extension, how much total you have in your account.