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I think KaiOS is probably the closest thing that you can find. It will also have a browser, but trust me when I say that on most phones it's so slow and unusable that you won't be tempted to use it often.

I had a similar issue for myself, only my strongly desired application was Spotify (instead of WhatsApp). Ultimately I weaned myself off Spotify through a phone with KaiOS and ultimately I took the plunge on buying the Light Phone II.

It's a real bummer that there are so many limited solutions for things like this. FWIW, the contracted software company that worked on the Light Phone OS put together a blog post detailing how they used Android to build their phone here:

https://medium.com/sanctuary-computer-inc/building-lightos-w...

> All of that is to say: when we refer to the LightOS, we are referring to “our custom fork of Android 8.1 that embeds a platform-signed React Native app as the default launcher” (amongst other drivers and low-level customizations).