You cant over inject. when it starts to just squirt the liquid out the other holes when you try to fill it your done. I inject mine while its still in the cryo bag. Let it sit in the fridge over night. Next day take it out and dry it, put your rub on it.
If you didnt like the texture, you probably put to much acid in it or it could be a fluke. you might just had a funny peice of meat. Apple juice, a little apple cider vinger, and some finely powder ground dry rub mix will get some good flavor in the meat.
It might extend the cook time a little. But ive never compared. I just cook it till its done.