A lot of the benefits of using a privacy OS is lost if you use many cloud apps. You must start with a goal in mind. What do you want to keep private from who? I've used iOS, Graphene, and CalyxOS. In my exp Graphene is the most private. iOS is very secure by default but you are sharing a lot of info with Apple about yourself. You are in their walled garden. Its a nice garden but you don't have freedom and maximum privacy.

I think one could use iOS in a more private way than most people do but this would mean only using open source app and avoiding many of Apple's services. If you want freedom from the cloud overlords avoid stock android. Lineageos is better than stock but Calyxos and Graphene have no Google services and have many hardening improvements.

The main difference between CalyxOS and Graphene is that CalyxOS is managed and backed by a non-profit org and Graphene is not. It is more of a grass roots open source project. Both projects are open source. Both are good. The other difference is in how they handle Google srevices. Graphene uses sandboxed Google services and CalyxOS uses MicroG which emulates these services(Services meaning the services required by many apps in the app stores). There are tradeoffs for both. I recommend Graphene to most people. Go to their site and look at their support for Pixel phones and pick a phone up. Its not hard to set up these ROMs.