Trailforks might be super helpful for you in this search. It's a website and app that shows mountain biking trails, as well as their difficulty, popularity, and open/closed status. If you take some time to look around the site and have any lingering questions, feel free to come back here and ask them.
I got into mountain bike after moving to Vermont a few years ago. Finding new spots to ride has been a really fun process and Trailforks has been my "guiding light" of sorts!
A friend of mine is actually mountain biking over in Michigan this week at the RAMBA Trails in the Upper Peninsula. He said it is amazing, so if that's nearby, you may want to pay it a visit.
Hopefully some other folks familiar with with mountain biking in Michigan will share their recommendations too. Enjoy your summer out on the trails!
At REI, we believe time outside is fundamental to a life well lived.