YK11 is excellent and one of my favorite sarms that I have used. II would watch Dylan's video to get a better understanding on ALL the things it can do!
It is my understanding that YK11 is extremely effective as a myostatin inhibitor as @DylanGemelli pointed out so that is what I would look at as a primary option!
There is really nothing today that exists as a pure myostatin inhibitor in the way we would like it to work. It would have to be selective so that certain organs did not just continue to grow.