Rec2020 “data in” to panel capabilities “out” is usually done through a LUT3D or close equivalent written at factory and usually it cannot be modified by user.
There is a chance that if you try to correct grey ramp through OSD controls on that OSD mode some colors maped by that LUT will be worse.
Also your device is not very good for this task.
If you wish to try to modify display behavior for consuming HDR content without a computer attached to it try HCFR. It uses ArgyllCMS as a base like DisplayCAL but more oriented to those tasks.