MTC is great and I’ll credit two semesters of their intensive course for making me functionally literate. But yeah, it’s not the cheapest option. I think for someone in the intermediate-ish zone we’re in, the most effective thing to do is find a dedicated tutor to help you with exactly what you need. But those are hard to find…I’m still looking.
In the meantime I try to improve the same way I improved my English - by reading, a lot.