I searched and searched and the only thing I could really find was stretches for pre and post workouts, not just for gaining the flexibility needed to hold proper position. TLDR below...

So I recently got into road cycling, have been mountain biking heavily for about 1 year now and decided to make the jump into road bikes. I have had my bike for three months and ride it to work almost every day, I take my mtb on rainy days. I have noticed that when I do group rides for anything over 15 miles or so my hands go numb. I have been told my bike is too small, and I will be upgrading eventually, this was just a cheap "see if I like road bikes" bike. In the mean time I recently got a smaller stem because I felt like I was reaching for the handle bars and this was causing some shoulder pain. I just recently had pictures taken of me by my wife while on my bike and I can see that I am not in proper position but I feel that is as far as I can go. What stretches would help so I can get a lower profile, I did read in my hunt that one guy recommends "riding on the drops as much as possible." I will start doing this on my rides but I was wondering if there was any particular stretches I could be doing to help me gain ROM.

TLDR: I am apparently not very flexible and am having trouble staying in proper position, what stretches can I be doing to help this. I am going to start riding in the drops more to help with this, is there anything else I can do?

Here are some pics for reference.