I have mentioned it on other threads, but I was in my mid to late 40's before my beard filled in enough for a full beard (I am 51 now). So I have been where you are now. For years, all I could do was a goatee and mustache. Not to say that your beard will definitely fill in more as you age, but it may. I do not think there are any magic potions, if I had one I would be rich by now.
We all have different genetics. Some of us get us male pattern baldness at an early age, some get a nice full beard in our teens. Others have to wait, and some may never develop a full beard. I am not sure beards and MPB are related, but my beard started to fill in more as my hair started thinning.
Make the best out of what you have to work with now. Create your own style, you don't have to fit the mold of "accepted" beard styles.