Did the same. Just mowed them down and kept the area cut. Easy peasy. Now if poison ivy was that easy.......
Get a rotary cutter and cut it, then overseed with grass if needed. You probably don't need to do much more than just cut it if it's just brambles and weeds and you may get a surprisingly okay yard just by continuing to finish mow the area once you've rough cut it.
Those brush blades work pretty well for stuff that grows straight up. When I bought my Stihl FS 130, I talked to the sales guy about what I needed to do. I had about a quarter acre of thick blackberries that were 6-8 feet high on uneven and rocky ground. He recommended, and I ended up getting a 24-inch hedge trimmer attachment. I can't find that same model on their website now, but it's not the Kombi version that they have with the extra shaft length. That thing cuts through blackberry canes like butter, even the random pieces of fence that I kept coming across. I could pretty quickly slice vertically and horizontally to chop up everything into smaller pieces. The only thing I'd say is that it was a bit of a physical job with that much weight out at the end of the trimmer.
Bush hog. The 1025R will handle a 4 footer easily.
