Looks like a beautiful site. Just looked at the pictures.
Agree, you can’t fill in appreciably against the trunks of the trees unless you left tree wells.
However with none of us knowing the entire topography of the area or other challenges you have, it’s almost impossible solve your problem and generic suggestions are all you’ll get.
Furthermore, unless you know the ground type and groundwater levels, many of these suggestions may be ineffective as well.
You need to find out what you CAN do and then what will work of those ideas.
