@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.
@ben_dw there's no real good way to install an app store on Graphene. You can install APKs manually of course, or you could install F-Droid to handle all that (I'm not sure what apps they have available) but it wouldn't be able to auto-update apps. Google Play is annoyingly convenient :p
@ben_dw in the long-term the Graphene devs do plan to develop their own app store, out of security concerns with alternatives like F-Droid, but that isn't a thing yet and won't be for a while.
@jonah that makes sense! Thank you!
@ben_dw ...but if you're looking for privacy from ISPs (mobile carriers) or your government, smartphones are inherently not private devices. Their design inherently leaks lots of data (like personal ID via phone number, and location via cell tower triangulation or GPS) that can't be easily disabled, if at all.
If that's your threat model an iPod Touch might be best, as it's WiFi only so you can disable all wireless communications if need be, and they're just as secure as an iPhone.