You can disable system apps as long as they are not marked as critical or persistent apps that must be enabled for the system to work. adb pm disable --user 0 <whatever> is stronger the ui app info disable button.
Yeah but as you said if they are marked as persistent (which for oem spyware would make sense) you cant.
Not all OEM apps are marked persistent. Especially on non-pixel devices, you can disable a lot of apps.