The t8 is fine for a 20 gallon. When you upgrade to a larger tank, you will probably need to get the t5 at that time.
If you can position the t8 so it is unobstructed, that is ideal. You mentioned before that you could slide the screen so that it was not between the light and the lizard, perhaps you can try doing that again with the new light. Make sure he can get 6-8" from the uvb for the best exposure.
The serrapeptase may help with the swelling. You can order it on beardeddragon.co
Since he is young and you are catching this early, you have a very good chance of reversing, or at least halting, the progression of the MBD. Best of luck! Keep up posted on his progress