At 38 years old, 5' 10" 180lbs, an admitted gym slacker desirous of gaining a few pounds of muscle and cutting your midsection...
...you should really only tweak your diet and hit the iron room more seriously. I know it's not the popular answer and probably not what you wanted to hear, but it's true.
With your frame, history and ambitions there's no need for aas here.
Best to you.