As you can see attached most of the field are automatically agregated, we don't have the choice of the way we treat the data,
We are looking for average reach per post, there is now way to do that
same for engaged users per post> .
it is unfortunately not possible to achieve with Supermetrics for Data Studio. In Supermetrics for Google Sheets, you could create a spreadsheet function that counts the number of posts (rows of data) and splits reach sum with that number.