@ben_dw security != privacy. I'd argue that there are exceptional security solutions for mobile devices (iOS and GrapheneOS primarily). Privacy is trickier, because it depends on who you want privacy from.

GrapheneOS as @moz mentioned is great by default because it doesn't come with Google Apps (and in fact you can't install them in a secure manner AFAIK), so if you're careful there's no worries about your data being shared with big corporations oike Google and Facebook.