What if the signal at an event is weak?
Paying needs a connection. At the counter, a code for one app is the lightest thing to load.
Updated
Paying needs a connection on the customer's phone, and starting a sale at the counter needs one on yours. When the signal is weak, choose Cash App, Venmo or PayPal on the counter's code screen: it opens that app straight away, which is the lightest thing to load.
If the venue has Wi-Fi, make a static Wi-Fi QR code so people can join it with one scan.