I haven't been on for a while but have a 7yr old husky whom has been diabetic for 1 year and thankfully well regulated. His diabetes was brought on by cushings which ive also been able to manage but its tough and keeps me busy. The cushings forces me to give him slightly more insulin. An infection or certain supplements will also raise blood glucose. A dog bottoming at 300 will still feel sluggish and his kidneys and other organs are working harder to combat the residual sugar.
For starters, try to get a CBC biochem blood test as a baseline to see where your dogs bloodwork is and his issues are only due to diabetes as many of the symptoms you describe are due to an unregulated diabetic dog but could also be something else. If your numbers are bottoming at 300, he needs more insulin and the goal is to bottom him at 100-150. My dog currently swings 125 to 240. I cant tell you what amount but you should at least try 3 more units given his weight and see where that takes you but I have a feeling he is about 6-8 units away from his target amount.
Keep his food intake the same day by day and exercise the same as well until you have a good understanding of insulin and its effects on food intake and exercise.