I don't think its possible to directly build and launch a plugin on iOS, I have done a fair bit of research and testested a few methods for creating iOS apps for years, and I am fairly certain even Unity uses Xcode as a conduit to launch the simulator/upload test applications. If you want to allow actual publishing of created iOS apps, apple makes you go through their tools.
I just went through the Juce process, and this is the same. I opened the AUv3SynthPlugin from their examples, and selected Xcode (iOS) from the exporter dropdown list at the top of JUCE and hit the button to the right with the Xcode icon, and then I selected AUv3synthPlugin - StandAlonePlugin at the top left of Xcode window, which default selected the iPhone 8 Plus simulator, and hit the play button, this launched the iOS simulator with the app and it runs and appears to work (though I have no idea what the app is supposed to do, it has a slider and record button, and neither appear to do much in the OSX or iOS versions I compiled and ran.)
This is really the only route to allow publishing or running on an iOS simulator, and would be a great start. And yes the iOS simulator works great for testing.
From there I have solid ideas of ways to allow rapid running and testing on an iOS device. MobMuPlat and Gemma both allow running PureData patches with customized UI objects on iOS using libPD, and csSpectral and csGrain iOS apps by mr.Csound himself Dr.B both run csound as the audio engine, which you seem to have figured out for the most part also.
so after an we do an initial test to proof of concept, we could look at the option of an app that works like your cabbage lite on iOS, where you can run cabbage .csd files and from there you could even consider the possible option of making an iOS port of the cabbage editor, and those might bring you in some app store $, but first things first, I think its prudent to test one of your examples >> Juce AUv3 >> Xcode (iOS) simulator.
thats the one thing these other platforms I mentioned with PureData/libPD never had, was any ability to turn your project into any kind of actual app/plugin for iOS.. they box you in to just loading your patch in their app, which is fun but doesn't give a route to a deliverable product. I am sure you can get both going and potentially generate some solid revenue from your work.