Your first step in getting rid of porosity is to clean your brass rod before using it. I use Scotchbrite and after making the rod nice and shinny I clean the residue off with alcohol or acetone (well actually I use lacquer thinner because it is convenient for me because I'm a painter).

My next recommendation for beginners is to not slot their chain stays. It is easier to braze if you are filling an upright hole with brass and it isn't leaking out the sides. It is possible to make the tab on the dropout longer if you like that look. You need to fill the chain stay only to the depth of the tab on the DO. That won't take as much brass as you indicated you thought you might need.

It isn't possible to give brazing pointers only looking at finished results. I can still tell however that you got the joint too hot. You want to keep the color of the steel on the joint red and avoid orange or even hotter. Here is a very common rookie mistake you might be making. Beginners have a strong tendency for their flame hand to move towards the joint as their hand holding the brass moves in as it melts. This overheats the joint. You have to fight this instinct and instead mvre you flame hand back as the other hand moves forward or flick the flame off the joint. It takes some real effort and concentration to move the flame and brass hand independent of each other.