Hi Lindsey;
You have
exactly the same situation that we faced when we bought two years ago. 15 years worth of 'Old Growth' Broom forest.
It was crazy tall and thick, and there were parts of our new property we couldn't even get to until it was cut down. The Man cut it off at ground level with a saw.
We chipped ours. It's a bit of a pain to chip, especially the twistier pieces, but we did it! We piled it up and let it sit for about a year or so, and boy does it break down nicely. It has made some really great stuff for building the garden with. If you have straighter pieces, they can go into
hugelkultur or buried wood beds whole, but they are hard to stack. That's why we decided to chip them. And I wouldn't worry about seeds. After 15 years there are a squillion seeds on the ground already. Nothing can be done about that but to keep it mowed, or let the animals mow it for you.
Biochar sounds like a good idea as well. I plan on making some, too.
Good luck! And let us know how it goes.
Cheers
Tracy