I need some ideas! Lots of stumps...

Hello everyone!
This is my first post in this forum although I've been reading a lot from you for a long time!!
I'm writing form Argentina. I need some help or ideas on what to do with this situation:
I've been consulted in a huge property here in Buenos Aires, located in the Parana River delta. This is a very wet and flat land.
The owner is starting to recover the property, since it was planted 20 years ago with a willow monoculture and then abandoned.
This year they started to cut down the willows to make some cash to do a perimeter earth mound which avoids regular/tidal floods, some access roads and some infraestructure.
They want to re-establish in the property, make it productive and take a profit instead of putting money every month, all the permaculture way.
I posted in "trees" because the first thing that shocked me, (apart from the size of the property) is that I found, by now, 75 acres of willow stumps set at 7x10 feet. And 250+ acres to go.
I cannot imagine where to start to deal with this.
The willow forest is very poor quality regrowth. Each tree has at least 3 or 4 stems.
All the stumps are, of course, regrowing.
I wonder in the first place what is possible to do with the field full of regrowing stumps.
Secondly, how to manage the remaining forest.
And last but not least, how to deal with such a big acreage.

Any ideas or comments will be much appreciated!!
Thank you in advance,
Lucas

Lucas I know nothing of willow but do you have some photos of your property to help others?

A good starting point with a completely altered ecosystem like this is to find out what is the baseline....that is, what was there before major human intervention? Especially, before human intervention leveraged by fossil energy? What was there before Spanish colonization? What yields did the land afford in those days?
The second question/line of reasoning has to do with having a plan in place as to what to replace an existing system with before you destroy it. What is that willow doing out there....what yields are possible from it? (fuel wood, charcoal, biochar, basketry, forage, soil remediation, water removal, etc. etc.) A very good way to proceed on a large landscape is to make small interventions, preferably close to human settlements and dwellings where they can be supervised; while leaving the pre-existing system in place on the rest of the property. Then later when several ideas have been tried on a small scale and the best among them discerned, expansion with these can proceed......

The 1st question is how to kill the willow, do you have to do daily cutting until it uses up all it's reserve energy. Maybe just grow vines on it like kiwi, grapes, etc until they kill it. Or find some animal that will eat it and overstock them until they kill and the regrowth. Once the willows are finally done, I would still run an animal system for a year or two. Then switch over to a more plant based system. You could also do fish system via pond