Is a Chinese phone's stock OS better than GrapheneOS for my threat model?

The question is: What exactly are you looking for? Privacy from Law Enforcement (LE) on causal stop and searches, that is, causal Plausible Deniability (DP)? Strong Anti-Forensic (AF), and/or General Privacy (GP)? If you are strictly looking at causal PD towards LE on the streets, then the Chinese phones could be a mitigation. However, without knowing your jurisdiction, you should consider it as poor PD. You should also consider, and take into account, LE may be aware of the SecondSpace/PrivateSpace on these Chinese phone. In your jurisdiction, is LE known for seizing devices if they suspect you are hiding something? If so, then you should consider GOS, as it has much better AF and GP.