Replacing OEM Android is only one part of the equation. There are many components that run with full privileges (EL2/EL3?) outside of Android and never subject to Android's sandbox. Well, at least until Google can force OEMs to stricter standards with their effort to KVMize those privileged executables [0].
If you do not trust the OEM, replacing its ROM with GrapheneOS / CalyxOS / LineageOS isn't going to help much anyway.
One right answer to this is fully open-source (hardware and software) phones like the PINE64 and Librem, among others.
I'm happy with my degoogled phone. There are so many great open source apps on f-droid. Still, it's 2021 and I need a few closed source apps on my main driver, so Aurora Store, MicroG, AdAway. Ok so I'm still using google servers for push notifications of the closed apps--compromises. I still have way more control and battery life. I love the idea of a linux phone, and maybe I could sacrafice the last few closed apps, but what I hear from users is that the hardware sucks. I want a nice phone.
If you do not trust the OEM, replacing its ROM with GrapheneOS / CalyxOS / LineageOS isn't going to help much anyway.
One right answer to this is fully open-source (hardware and software) phones like the PINE64 and Librem, among others.
[0] https://lwn.net/Articles/836693/