Seized Quill Stem bolt

Probably over-tightened rather than rust as it was new and not fitted long ago. It wouldn't budge and then when I tried a torque wrench on it the bolt rounded out. I tried screw extractors with the drill but the drill lacked the torque to shift it. I tried a tap wrench and eventually have resorted to drilling the bolt out but after 3-4 hours of effort there isn't a lot to show for it. Next option is to cut into the stem and sever the bolt. I am just worried because I don't have a clue what I am doing and don't want to make a really shit situation worse. I seem to have drilled down almost to the depth of the bolt head (I have an unfitted identical stem) but I am not sure the drill bit is wide enough. Anyone have experience? Should I take it to a bike shop? I just thought this may be beyond many a lbs (mine is an Evans)

Not the weather was this sort of crap.

Aluminium stem where the bolt sits directly into the stem
Or steel stem with a separate little cap it sits in?

If ali I’d just cut it, should be an easy job and then bash the bolt/wedge downwards to free everything up and remove.

If steel maybe continue drilling, try make it through the cap then as before, bash downwards to free.