Shadows are primarily decided by exposure, as you have found out.
But, developing the film longer will change shadow detail a little bit. By diluting your developer you are forced to develop longer to get the same negative contrast, so that will, by design, give more shadow detail.
Have you tried simply developing your film longer in order to get the 'normal' contrast that you get with stock developer? Or somewhere in between? If the developer exhausts it means it will not develop your film any further.
But at the same time, if you're happy with the results you get now, change something, and then don't like it as much, then you didn't gain anything. It's up to you, really.