Although I definitely do not recommend buying an iPhone, sometimes we get stuck with them. Especially if you try to always buy used phones, which is a good idea. A phone with LineageOS and without installing the Google apps is a much better choice for a privacy phone that gives you choices, and this is a good guide for better privacy, but if that involves buying a new phone from Amazon as opposed to making do with an old phone that nobody wants, then some of us are stuck with an iPhone. Apple probably tracks you less than Google, because tracking is Google's main business model, whereas Apple's business model is to try to force you to use Apple services and buy more Apple products.
So the idea here is using an iPhone minimally, as just a phone plus any offline apps you find useful (Camera, alarm clock, etc.). It's less convenient and you have to jump through a ton of hoops because of Apple's limitations, and I can't guarantee it will work, but in the end I was able to get a more private iPhone 8 than I would have by doing things their way.
When you set up the phone for the first time or after a factory reset, don't create an Apple ID and don't get an iCloud account. The interface makes it non-obvious to avoid creating one but it should be possible, at least on the older models.
If you want to access files from your computer on your phone, instead of syncing them with cloud storage, you can sync them via USB cable on a Linux computer. Note: on iPhone, text files are read-only because it doesn't have a text editor, so I basically live with viewing my files on my phone, and waiting until I'm at my computer if I want to modify one.
First install Firefox (or some other app that lets you transfer files from your computer) on the phone from the App store - I believe this is necessary so you have a folder to copy files to in your Linux computer, because all files that get transferred are associated with a particular app, and iOS is so badly designed that there's no default app to associate the files with.
Plug in the phone via USB cable in your Linux computer. It should appear in your file manager (for me, Thunar, and it appears under Devices).
Go in the Firefox folder, then go to File -> Open in Terminal. In the terminal window that opens, enter the command pwd to show the path of the files on the phone.
Write a script using rsync to copy files from your computer to your phone. E.g.:
rsync -rt --modify-window=1 --progress --delete ${HOME}/phone_folder ${phone_loc}
where ${phone_loc} is the folder where the phone's files are stored which you found via the above process. Note: A second call to rsync to copy from the phone to the computer could let you sync the files, but since files on the iPhone are read-only, there isn't much point in my case.
Note: Pictures from your camera appear on a separate device in the file manager and have a separate folder to copy the files from. Rather than syncing them to iCloud to get them on my computer, I move them to the computer via the USB cable and then delete them from the phone.
Note: For music, I copied my music to my phone once via iTunes in Windows, then don't need to sync it anymore. I can't find another way to copy music to my phone's Music app via Linux.
For contacts, instead of syncing them in the cloud, you can just store them on the phone, and export them each time you edit or delete one. Then that exported file can be backed up to your computer as part of the above script. To export, it's a bit non-obvious: You have to go the contact list in Contacts, long press on the list, Export, then scroll down to find the Save to Files opion.
Note: this is for contacts that you call, i.e. a speed dial. You don't need to store email addresses in your contacts, email can be done on your computer and a separate address book of email addresses can be maintained there. You can still use your phone to receive emails and even reply to emails with a short reply without needing an address book of email addresses.
Note: to initially import your contacts from a .vcf file, you can put the .vcf file in with your files and then click on it in the Files app. However, I ran into some problems doing this: some contacts didn't get imported, and if you import them twice, you get a bunch of duplicates, and there's no way to delete all your contacts and try again. That's why I only did the import initially, and only do exports on a regular basis.
Instead of syncing a calendar you can just use the Alarms app to set reminders for yourself of events. You can give them labels and repeat them weekly. You'll probably still want a main calendar on your computer or paper which is offline, for planning events, and just use the phone for reminders. Unfortunately I haven't found a way to sync the calendar via a USB cable.
Avoid even connecting to the internet at all, just using a basic phone plan with no data and don't connect to wifi. That means not receiving emails or other messages on your phone and not using the web browser.