cmahb How to change WebView settings in a private profile?
You don't... This question has been answered and discussed many times, here is one example of a recent answer:
(Please consider searching the forum prior to posting a duplicate).
GrapheneOS Vanadium WebView is the only one permitted by GrapheneOS, similarly to how the Chrome WebView is the only one permitted by the stock Pixel OS and Google Mobile Services operating systems in general. It would be a huge violation of the app security model to permit arbitrary WebView providers since it's given access to data of other apps through them using it as part of their app's implementation. It's an OS component. There's a system for installing other OS approved WebView implementations but that's meant for use cases such as using a WebView provided by the Chrome Beta channel rather than a third party one.