Old fields overgrown with woody brush

I was given the opportunity to cut back a 15 acre field that was once a productive hay field. It has been out of hay production for realistically 10 years.
The field has been invaded with what we call "Russian olive" (may be incorrect) and other typical trash like golden rod, broomsedge, winter olive, blackberry, etc. These Russian olive invasives are about 10-15' tall now. They are as much as 3-4" thick at the base and spaced ~10' apart on about a 4 acre patch of the 15 acre field. Rest of the 15 acres are light brush, weeds, grasses which are easy to mow.
Customer will pay me my asking hourly wage up to a point (I can't spend days clearing the brush, I'm guessing one full day will be his "pain tolerance"). My Woods BW180XHD was designed to cut brush up to 3.5".

The upside other than being paid to clear brush is he will let me hay them once they are cleared. The field was once productive timothy fields. I can attest to that because I remember them being hayed. Another bonus is that there is a second, 5 acre field that has been mowed annually, so the smaller field is light invasive weeds. I think it could be mulch hay next summer. Woody field is more questionable as to when it could be hayed.
The downside is the small stumps and woody brush leftover from cutting. I think it might involve ripping the small stumps out before I can take a disc cutter over it without damage. I also realize the shredded wood from the brush will be in the field and get in the hay. Not sure what to do. Thoughts include pulling my 8' York rake over the woody junk into piles at fields edge and using loader bucket to dislodge small woody stumps.

Acreage is tough to come by. 15 acres of even mulch hay is not a bad asset to have.
However, I'm afraid of gearbox damage or other damage to mower for a small paycheck. Slightly Concerned of loader damage to dislodge small stumps. Love the thought of restoring 20 acres into a more meaningful purpose than invasive weeds. Access to fields is good and they're fairly flat. Owner seems like a good person, but overwhelmed with his greenhouse nursery business and would like someone like me to take the fields off his hands.

Any thoughts/suggestions? Sorry for long windedness

Old fields overgrown with woody brush