What do you consider long term dietary control?
I have been low carbing for decades, very low carbing for... um... 5-10 years, keto for 3-5 years, and carnivore for 9 months now.
I have normal blood glucose levels (below pre-diabetic), but it took going keto and carnivore to achieve that. Low carbing in the 30g carbs or above range had my blood glucose levels in the pre-diabetic range or above.
During the last few years I have trialled both berberine (and more recently metformin) but neither seemed to offer enough benefit to outweigh the gut consequences - and the hassle of remembering to take the dratted tablets. I am currently on no D medication, but may trial Metformin again in the future as another experiment. However that won't be for some time, since I am currently dealing with other gut 'stuff' and I'm not going to put metformin into the mix.