I mean, exactly how invasive are default operating systems? (Like Windows, Mac, Chrome OS, Android, iOS) Do they log your keystrokes, log passwords, capture screen, upload your photos, videos, or audio? (Assuming you aren’t a target of government) Is it even possible for the average person who doesn’t feel comfortable messing with installing operating systems to have any privacy?
Linux is not like a custom ROM, it’s just an OS like any other (but many distros do respect your privacy). But yeah, custom Roms are the only way to have privacy on phones other than just using dumb phones. Support open source projects and manufacturers that make devices that use open source software!