I'm a robosats user myself, and the main problem I encountered for making it work for this usecase is speed, and streamlining the experience from scanning the merchant QR to the point where the taker makes the payment. By using cashu/custody, we can take out a lot of the variables that LN might have, and do all the clearing inside the mint. It's separating withdrawal from the actual orders, basically reducing the steps and shrinking the time needed for a payment to the merchant to complete. I think payment speed is crucial when paying a merchant, so that is what openpleb will focus on.