I have a plain Cordova app and when I run it on iOS 14 beta the app gets stuck on the splash screen and there are few warnings/errors logged in XCode but I am not sure wheth
You should upgrade to Cordova 10 and then remove
<preference name="WKWebViewOnly" value="true" />
<feature name="CDVWKWebViewEngine">
<param name="ios-package" value="CDVWKWebViewEngine" />
</feature>
and remove
cordova-plugin-wkwebview-engine
Then
cordova plugin add @globules-io/cordova-plugin-ios-xhr
and setup the flags like you would for wkwebview-engine.
You also have to upgrade your splashscreen plugin for the new splashscreen image format. Read https://cordova.apache.org/news/2020/07/21/splashscreen-plugin-release.html