So I'm guessing you want to update Project_Data with values from data.
First I would use an after-commit handler on Project for this and pass parameter Project. Because I only want to update/sync Project_data if the commit on Project is successful, not on beforehand.
In your microflow place action ‘retrieve Project_data over association’ (make sure it's there using findcreate pattern or place empty check) and add ‘change object’ action for Project_data.
Change the desired attributes where you set the value with corresponding $Project/Attribute.
Be sure to commit the Project_data object and possibly refresh in client if needed.