As a T2D (not using insulin) who has used Metformin for about 15 years now, I do not suffer too much from the gastric overflows and tolerate the max dose (2g / day). However, I have started to reduce my dosage of this drug as I find that my Low Carb llifestyle is a better way for me to control my bg's. I also suffer lactic acid build up in my calves, which I am told may be due to this medication. So far I have reduced by 2 tabs (i.e. 1000 mg) and my bgl average has risen by about 1 mmol/l and I find I spike a bit more than I was doing. So this is in line with the reported finding that Metformin has minimal effect on reducing bgl, and each tab will only drop bgl by a small amount. (much less than a single weetabix will spike it indeed) So I believe your high bgl levels are not really due to dropping the Metformin. I suspect your diet, as others have commented too.
Metformin is prescribed to us diabetics (both T1 and T2 etc) not as a bgl control drug, but more for the added protection it gives in reducing heart and circulatory problems and this is demonstrated to be good for us by several proper studies into long term use of the drug. Mortality rates are lowered by this drug, especially those nasty CVE's that diabetics are apparently more prone to,
I see some here are suggesting you use a ketogenic diet, but in my experience you can get good results simply by cutting down on carbs and processed foods. The keto part could follow later on. I have tried keto, and found it beneficial, but now I follow a higher carb level diet based on the Mediterranean diet which I can be more relaxed with. It is also a diet that my family can share to their benefit too, and it has certainly restored some of my insulin response so I am more carb tolerant now. Having said that I broke my rules last night and ate an Asda pastie, and had a nasty spike, so I am not cured yet. There are other diets too that can help you, so study the info on this forum and find what suits you best. The real answer is to keep a food log, and eat to meter to remove foods that your body finds intolerable.
Thank you Oldvatr really good advice