> I would hate to give my potential tippers a way out by letting them leave after scanning a QR code and then not using it because they are lazy, which they almost certainly are.

It's got nothing to do with laziness. Tips shouldn't be expected, and the difference between a QR code and a tip jar isn't going to change whether people think the service was deserving of a tip.

A better answer to cashless tips is just to allow customers to add it on to their total and collect it with the card payment. It's done that way in increasingly many restaurants in the UK and EU, especially with the decline of cash and a growing preference for contactless payments during the pandemic. No need for yet another service when the existing stuff handles it.