I can only tell you what I've done on our 61 acres.
Pasture had been over grazed by horses, then empty and run down for years, had exactly what you are talking about. Lots of locust groves, both black and honey, red cedars of every size, huge groves of blackberries and weeds everywhere. Blackberries where well over my head sitting on a tractor, and locust were generally 12-15' tall. You say you have like 100 trees? I had them in the thousands. Plus hundreds of downed and dead trees. I had a couple of old Ford tractors with no FEL, 6' bush hog, box blade, chainsaw, axe and a trailer to pile wood on to take to burn.
I bush hogged the blackberries not knowing any better, cut cedar and locust off at ground level with an axe or chainsaw (which wore out several chains). I don't bush hog trees unless they are a quarter inch in diameter or smaller. I mowed 3-4 times a year for the first 2 years. Blackberry tried to come up again, I mowed it, same with locust, locust is soft and easy to mow when it tries to come up again. I'm talking locust groves here, not 1 or 2. Most blackberry patches were maybe 30'x30' average and there were tons, just dirt after mowing because no light for grass to grow.
3rd year, hardly any blackberry or locust tried to come up (no spraying, NO stump treatment), they are well under control now but I do mow quite a bit, can't stand a weedy pasture. Still have plenty of weeds like ragweed but it's getting better. Bare spots have filled in with summer grasses and clover (did not plant one seed of clover, broadcasting fescue was useless) and rotate the cattle through when they get ready to move.
Wish I had before and after pictures. I have to say, if it takes 3 years for a chemical/mowing to kill blackberries, or it takes 3 years to kill them by mowing alone, I think the chemical company is getting rich off of you. I tried test spraying for broadleaf weeds one year on some small patches and I will never use chemical on my pasture again. I don't know what I was thinking. I'm not one of those nature nuts, but it's just not for me. I did not get everything done the first year, and still have sections I work on to this day (burned trees this week in fact, and last week, and the week before actually.....) It's great if you like manual labor and don't want to pay a gym membership.
Good Luck