if you cant get it off and your only choice is to weld... you dont HAVE to weld a nut on. you can weld a small bar, perpendicular to the stud, and then twist it off via leverage. trust me, ive been through that shit TOO many times. and its a hell of a lot easier to weld on a bar (if there is not enough threads for a nut of course) to a snapped stud.
woops just read the part where you said 8-10mm threads left. then yeah welding a stud would be easier. screw it on, weld it on remove it off

use a mig welder if you can, use the same material as the snapped screw for the stud and the as the wire feed. file or sand the stud to remove any oxidation, get a good nice clean surface so the screw and stud dont oxidize under tremendous heat

if you cant find a mig welder, a stick welder will do, but more trickier.