The only thing that I can think of that would be helpful in this situation would be basal insulin (Lantus). This may or may not interfere with fat burn, you really need to talk to someone who has a lot of experience with basal insulin during contest prep or experience using it to mitigate beta cell damage as a result of heavy GH use.
If berberine isn't making a big difference I wouldn't have high hopes for metformin since they are similar in terms of efficacy but it would be the next step if berberine or other GDA's aren't helping.
Basal insulin can be useful for minimizing damage to beta cells in your pancreas to reduce the chance of developing t2d from being insulin resistant for too long (split dosing GH for too long, being too heavy, eating absurd amounts of carbs for too long etc). Contest prep is a pretty unique set of circumstances and even though I have experience with basal insulin, I have no clue how useful or detrimental it may/may not be for you.
I don't know how detrimental dropping or significantly reducing the GH would be to your condition, but I wouldn't dismiss the idea if you can't get it under control by other means.