Provided you are still only taking Metformin and you felt fine, you're f
The 5 to drive is for those on heavier drugs than Metformin, like insulin. On Metformin you are fine to drive on a lower number
provided you feel fine. If you feel off or rough, then don't drive, irrespective of anything else. That last bit is just common sense really.
My numbers now seem to run naturally low, and I am often down into the 3s and feel fine. I don't take any medication at all.
Just listen to your body. If you are beginning to run this low regularly, then it may be possible to reduce your medication, if that would be what you want to do.
Well done. The hard work is paying off.