for some hard gaining people manual exercises aer not intense enough to work, and some people cant perform them for a long time without getting tired. Jelqing and manual stretching (standard) doesnt work for you and it didnt for many others, dance and myself to name two.
I recommend trying hanging over pumping, it has to have the best success rate surely, and the variables of time and intensity are set and very changeable so it is the most versatile method too. Can you say exactly how long your penis has been under tension and the exact tension on it with any other exercise? No.
Also you can work or whatever while you are hanging. It takes a long time to become proficient and its very time consuming but im sure that this is the way to go for you.
Feel free to ignore my advice if you want! Im just offering a suggestion for you.
Good Luck SS4
hmm work while hanging that would be nice but how noticable is the devise in your pants