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

I have a similar situation to deal with. Chinese private space implementations do work but if an enforcer was properly instructed beforehand, then they can and they will find out that particular functionality is active (for example some manufacturers leave a special shortcut in notifications section for quick access to main space etc.). It’s a consumer solution in the end, so it is pretty basic. These implementations work for protection against basic criminals to a certain degree, in a heat of a moment so to speak but it’s joke for an experienced government man. IMO your best bet is a second phone with GrapheneOS for any sensitive activity in WIFI-only mode, that is permanently residing at your home.