I’m one of the very few rare Type 1s that ever took anything like Metformin. I say that because I started taking it in 1962, and in those days they only had Phenformin, a related (and apparently more dangerous) drug in the biguanide family.
I was actually taking it because my endo had tried every combination of insulins and diets that he could think up, and my control was nowhere to be found.
What happened was that taking ONLY 2 Phenformin pills with 2 medium speed insulins (Globin, no longer available) I went from being his worst patient to being his best.
Also what I learned way back then (or sometime in between?) is that Metformin really slows down the liver’s ability to release stored glycogen (which turns back into glucose immediately) which is normally done to support our glucose needs when the stomach isn’t supplying immediate needs. Our diabetic bodies (at least in some cases) output too much glycogen because of individual characteristics. My situation was fairly rare back then, he had 5 more Type 1s after me that did very well with this treatment. It didn’t help his other Type 1s AFAIK.
The other thing he did was to reduce my carbs and increase proteins, which also helped since it required less insulin and kept me more stable. Still doing that kind of thing after a total of 62.5 years of Type 1.
Good luck, hope this helps.