You seem to be bringing in artistic judgments, which aren’t necessary.
If the dynamic range of your scene is less than that of your sensor, then you can ETTR. The premise of ETTR is that you’re not going to any clipping.
If you dont have clipping, then you don’t need to assign the luminance value of “mid tone” to the sun-lit grass, you can decide this during post.
If the dynamic range.of your scene exceeds the dynamic ranger of your sensor, and you have no.means to control that contrast, like a fill flash, then ETTR is not as applicable and you must select which values you want to record. In this scenario, you will need artistic judgments, as this will determine what detial you can render in post.