If you are this paranoid, you shouldn't be carrying an electronic device.
I'm in the camp of pragmatic resistance to surveillance. I use browser plugins to block ads and cookies where it doesn't get in the way of reaching the content I want; I use Signal for messaging even though almost none of my recipients do; I disable location services except for things like Maps that actually need to know where I am; I turn off all the spyware I know about that's built into operating systems; etc. I'm not a tin-foil-hat-wearer; I'm not doing anything illegal that I need to hide; I'm just trying to push back in a small way against the erosion of privacy and rights that permeates everything electronic.
But the parent isn't paranoid. They really are watching. And we shouldn't be so complacent.
This person is so paranoid, that they believe that a cyberweapon developed by a private company in Israel that uses previously-unknown bugs in the most sandboxed messaging system you can get on a phone are going to be deployed against them, so they should not use the calling, texting, or any other "phone-like" functionalities of a phone.
They then distrust that the End to End Encryption is in-fact End to End, and then think that using Signal or something is more secure, when if a bug in a system more sandboxed than Signal was found (iMessage, which has BlastDoor which Signal does not have), it is more than likely that Signal has it's own zero-days in it, so you shouldn't be using that either.
That's paranoid, and if you are that paranoid (which, maybe you have a reason to be), your solution isn't well thought-through. You shouldn't be using a phone if you can help it.
Fun fact: having systemwide location services on, even if you don't enable it for any apps, means that your location is sent in realtime to Apple/Google at all times (via Wi-Fi triangulation data). It's not just passive GPS reception.
If you want actual location privacy, you'll want to leave location services off systemwide on your smartphone, and consider getting an offline GPS receiver device. Good car satnav devices from China are like $60 now, and include continent-wide maps, though you lose realtime traffic info, being offline.
If you just don't want to be watched, either by people or algorithms, and have a rational understanding of what tracking/surveillance you are under, and you are actually not paranoid.
There is a degree of rational fear, rational expectation of being tracked. Your degree of fear though is irrational unless you are, in fact, a journalist in an authoritarian state.
You are saying that you are so paranoid, you don't trust iMessage to be End-to-End Encrypted because it has zero-click exploits developed as part of a cyberweapon that is explicitly targeted against high-profile journalists. You then think using Signal or something is more secure, even though if this was pulled off in iMessage (more sandboxed than any other messenger security-wise), your other messengers probably are also flawed and you shouldn't use any of them.
In fact, you shouldn't use a mobile device. And maybe for your situation, that is right and rational. But for most people, it's not.
You don't have to be interesting, you just need to exist to be caught up in the dragnet.
