So that's fine and all, but if you've lost that trust in the platform itself, you can't trust any E2EE; it's not as if browsers somehow fare better in that analysis; they are in fact strictly worse.
browsers do add one more layer of trust when thinking about the OS, but any mobile app also does?
Browsers can run on platforms that don't have secret baseband firmware, obligatory auto-updates, etc. etc.. Obviously a layer on top of a specific platform can never have better security than that underlying platform, but you can run a browser on a better platform.