Obviously with iOS this won't work, but then again, iOS devices aren't cheap enough for this to be a reasonable approach anyway.
Device #1: Buy the cheapest device you can. I give my friends specific recommendations. Invent a new burner gmail account for it. Install whatever you want off the Play Store - obviously, use all the normal caution to avoid any malware. Never put any personal info on it, so never put a SIM card in it, just connect over wifi.
Device #2: Buy a nice Android device. Never sign in. Put the SIM card in this one. Side load apps and update them by extracting the APK from device #1. Export your contacts from Google and load them manually (and save a backup). Access your email using IMAP from a third party app. Backup your photos using a third party app. The point is _not_ to use Google Pay, Google Assistant, and Google Cloud.
• Google Maps works without being signed in
• Google Play Store will update Google Maps (and the default apps on the device) without being signed in, but it does use dark patterns to try to get you to sign in
• Android OS updates will work without being signed in
It is more work, no question. It works for me. Your mileage may vary. Disclaimer that I'm posting in kind of a hurry, so I'll just admit the details here probably don't make sense.
I want the same benefit as the desktop PC, specifically that I own it, control it, and don't have to sign in to use it.
Sent from my OnePlus 6T running LineageOS.
If I am going to go with a different OS than Android, I'll get a Librem so I'm paying the company that made the hardware AND software, with incentives aligned for me to be a supported, acknowledged user.
Also, once you've done all this you can download APKs from Google Play Store without needing a burner phone. They've had extensions that do this for some time. Might as well just do it from the computer and or use an alternative app store.
If you really don't want someone to sign in to Google services, then there are ROMs made specifically for that. Have them use CardDav and CalDav. Use email with IMAP, etc. The route you've chose seems way too convoluted for no purpose other than the illusion that you're somehow unidentifiable.
By purchasing a burner phone, I am definitely traceable by law enforcement. You want to be untraceable after a crime - I'm not solving that problem.
Your other suggestions are fine ideas, there's not a specific right or wrong way to do it.
