GOS21606 If you want privacy use your phone as little as possible. (also a mindset)
I used to think this way too, with all the many sensors a phone has and the many purposes we use it for, it should be a privacy nightmare.
However now GOS exists and I'm back in charge, even Google only gets to see as much as I let them, with all the permissions, scopes and sandboxing GOS gives us. Nowadays my mindset is: If something has to connect to the internet, I much rather have it run on GOS (as opposed to Windows, macOS, iOS, wearOS, any Linux distribution or AOSP based system).
Most of my workflow and data is processed on GOS, nicely compartmentalized in user profiles, thoroughly restricted to what I want to allow and nothing more. We don't have that kind of power elsewhere (Linux distributions maybe, but at a big security cost).
I agree on the card and cash approach though. If you can contain something better separately, don't use a phone for that. But for the rest, a GOS phone will increase your privacy if you know what you're doing.