I believe 40 mg is the smallest dose in tablet form. 2.5 g of Metformin is more than the max dose for that med (2,000mg). You could buy a pill-splitter device at the pharmacy, and halve the 40 mg pill? I have done this in the past
I was on 2g Met and 40mg Glic and I too get readings in the 3-5 range usually in the evenings, Again, I do not get hypo symptoms at this level like I used to. I think I am now fat-adapted, and ketosis is kicking in to protect my brain functions and I eat high(ish) fat to help fuel my body when glucose is low. I find this to be a relatively stable situation to be in, and since I no longer drive, I have no inclination to drop the Glic. I have recently halved my Metformin to 1000 mg and this has raised my sugars back up to the 4's.
One warning though. If I drink alcohol in the evening, then I make sure I eat a carby snack at bedtime, to make sure I am not going to get into difficulty in my sleep. It is my safety net under those specific conditions. The other tip I share is that in my case it is normally my 2 hr PP after the evening meal that drops me the lowest, I normally find that at 4hours PP, my sugars are rising naturally and this ties in with the effect of the Glic only lasting a few hours before being excreted which is, in a way, added protection against hypo. After 5.4 years of Glic use, and Low Carbing, I have never had a hypo where I have needed to seek assistance.