I know that GrapheneOS has a lot of security features that make it basically impossible to compromise your phone. And that it has a lot of control over permissions and has some privacy features. But it also has a Google Services compatibility that would allow you to use Google services, which would allow Google to harvest a lot of data from you; much less data, but still some. Now OSes like CalyxOS or Lineage have microG which in addition to giving you the APIs, it uses less battery and has the ability to use Mozilla network location to stop google from getting that data. CalyxOS and Lineage don’t have the crazy hardening modifications that GrapheneOS has, but Android is already crazy secure compared to something like Windows or Linux without a properly configured SELinux or AppArmor. Why have Graphene over Calyx?
This is very dangerous thinking. No software is perfect; there are always ways to get in.
Now, in practice most people aren’t victims of targetted attacks and even devices with dozens of known local privilege escalation and remote code execution exploits won’t ever be attacked but those who will cannot rely on “x is basically impossible to compromise”. It takes layers and maintenance to actually be somewhat secure.