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 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.