You should be able to download the iOS 14 beta profile if you go to https://developer.apple.com/download/ while in Safari on your phone, but if you're having difficulty there here is an alternative site where you can get the profiles betaprofiles dot com.
kmt901
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You will have to go back and forth between Xcode GM and Xcode beta, apps can't be submitted to the App Store from Xcode beta. Having a separate branch for iOS14/Xcode 12 is pretty standard! Just make sure the changes you make in your main branch are continually getting merged back into that branch too. Also new additions to SwiftUI are backwards capable with iOS 13 so you won't have to necessarily bump up your deployment target to iOS 14 when fall comes and you can always wrap iOS 14 features in an available conditional.