If you only care about security, you should keep Play Services isolated in a separate profile. That way, even if there happens to be a memory corruption vulnerability in Play services, which isn't caught by hardened_malloc or the hardware MTE in newer devices with ARMv9 chips, the rest of your system would still be safe, since Play services aren't running as root, and in order to compromise the entire system, there would need to be a privilege escalation vulnerability in all of Android, not just Play services.
And you know what helps reduce risk of exploit? Smaller codebases.
Why does CalyxOS include the F-Droid privileged extension then? It's yet another component running with elevated permissions and unnecessarily increasing attack surface. Why does it include Google's eUICC component with elevated privileges and no proper sandboxing?