Seems justified. If anything you practiced restraint by not killing him. If I was in your place I’m not sure I would have stopped because I’d be too scared that the wound wasn’t enough and that he might get up at any moment and start attacking me again.
No. He was trying to kill you. Imo any amount of force and any target is justified in that kinda situation. Even if you managed to slice off his balls it’d be justified imo. Granted, if he disengages and you feel safe, then you should stop; I’m just saying that, in the moment, I’m not sure I would have felt safe enough to let him disengage.
Edit: the throat is far more vulnerable than the belly. Smaller target, but significantly easier to damage something in a fatal way.