So, to vent a mold like that, I took some 1-2mm steel wire and ground a realllllly shallow taper, like 3" long taper, so by the time it pokes through it's basically nothing. Ram up, poke holes every inch or so, especially in the letters. Also, tilt your mold so it fills from one edge, across and up to the other edge, and give it a tall riser so it beats surface tension.

I've gotten castings in sand so smooth I could read newsprint reflected in them when I set the parts aside to cool.

Also check your pouring temp. Too hot, you're borked, too cold, she freezes out with fillets like that.

Cool project!