There are very few benefits to being a full-time employee that you can't obtain yourself. Plus, having a wide portfolio of freelance clients helps insulate you against business downturns and future age discrimination.
Granted, it's a lot of work to curate a good client list and network yourself. But it's actually the same amount of work as jockeying for promotions and good projects at a corporate job.
And there is some risk involved in working for yourself. But at a corporate job there is similar risk; you are always one bad quarter away from being laid off.
Working for yourself IS a career. Consider fortifying your position and staying the course.