Now looka here sister, I grip thumbers a little different. I extend the index and middle finger out on the bottom of the flight plate as if I were kinda throwing a baseball(or my inverted FH grip), which gives me a more comfortable grip and more angle control over the disc. Just make sure your thumb is comfortable and has good control/pressure and the disc can pivot. I think 300' OH might be pushing it for you though. OH shots can be rough on the shoulder especially when not done properly. I used to pitch and try to limit my OH shots to only when its the best option. I don't even practice them anymore, but I'm totally comfortable with them and they are very predictable shots.
Coda throws a nice thumber, its labeled as tomahawk, but I think its a thumber, looks like flight plate is outside his body. Anyways tommy and thumber are very similar in technique. Look how vertical his shoulders are and his foot pivot: