I'm not sure you can guarantee a great user experience on a free, open web with user privacy and choice on a device you don't have rights to because it's completely locked down beyond your control by the vendor.

There should be a way to run a device that's not controlled by Google, Apple or Microsoft; not just because you're "paranoid", but because you'd like to learn and contribute, and maybe come up with some cool ideas yourself.

Point in case, because it's open-source, you could work on those browser features you'd like to see on mobile systems.