>Nowadays even cheap phones come with a battery you cannot change - which means you can never be certain the phone is REALLY turned off.

Scary indeed. Time to make wire mesh etui...

>>and most of the instructional videos that I find are not about removing Android from a phone, but rather about running Linux as an app under Android - which is not the goal.
>
>Hmmm, trouble is that even other OS will try to run android apps and those will have ears inside their layers. I am not certain a Linux OS will be much better than Lineage or other AOPK fork.

That's why it's only my second option. Even so, at least under linux you can have a real firewall. The last one which worked properly under windowses was something called SysGate or some such silly name, which was then bought by Symantec and defanged into ZoneAlarm (so it never got built for 64bit... the 32-bit version should still work). Any other firewall after that either considered ALL outgoing traffic as regular, or at least anything coming out of registered software (like windowses themselves) and it was either impossible or very very hard to set up to stop such things leaving your box. I still have a screenshot of how any run of a .msi file calls a server in Redmond. Every one of them, even those you have built yourself, using Wise or some such tool.