KingShadowUK As mentioned earlier, as long as Samsung refuses to support verified boot and other security features for third-party operating systems (source), GrapheneOS can't support the device. That is up to Samsung.
As for comparative security, it's hard to say in a definitive sense. This summer some information was leaked about which devices Cellbrite could break into, and according to that information Pixels were more resistant than Samsung's devices (source; see "Table 2: Android OS Support Matrix - Locked Devices 7.69.5"; note that there are multiple "Table 2" items).