GH is a great addition in your situation. You're already very very lean and GH makes it harder to store extra calories as fat. Being that lean, you will also have much less issues with water retention and insulin resistance.
Age 30 is where you start to see the GH decline in most people and is the ideal time to start for anti-aging purposes. Don't wait until you have wrinkles, thinner hair, poorer sleep and worse body composition to start it, ideally.
As long as you still have some gains left in the tank, you can still put on muscle with 150 mg testosterone, especially with HGH, but that depends on how close you are to your genetic limit. You will have to raise the calories to a moderate surplus and practice progressive overload for best results, the GH will also help keep those gains lean.
Generally, people do best with 2-5 IU daily (AM or PM or split twice daily) or the equivalent dosage but split between monday/wed/Friday. This is the range where sides are manageable for most people with results you can see and feel. Everyone responds different. For me, splitting twice daily makes me feel like crap after a few days.