If it were me, I would weigh the trade offs of reducing your medication. What would be your reasoning for reducing your medication? Are you having issues with its side effects?
Getting down into the 6s is tremendous progress, but it sounds like you've utilized every approach available (to you) to get there. If it were me, I'd be concerned that my levels would increase as a result of reducing the metformin.
Again if it were me, I'd be asking to reduce my medication if my levels were in the 4s but not before getting to that point. At, say 4.5, you have a lot of room for fluctuation if the metformin really is working well. In the 6s, I'd be concerned that the metformin would push me into a range I wasn't comfortable with.
Short answer: if it's not causing any problems and it's working well, why change the equation? If it is causing problems, decide if they're worth the chance that your levels might climb.