I'm in a similar situation and considering the $0 IAP for starting the app. But, it's not really nice from the user perspective. My better option would be to use my backend to validate and start the app for a trial, and present the purchase button as IAP if and when the user decides to buy it. This would avoid 0$ purchases that feels a bit strange imho. Not sure if this is allowed though as the language on the app review rules it's not super clear to me.