How can I reduce a tool's shaft size to fit a drill chuck?
Big mistake reducing that shaft.
First, you have a serious misfit between the application and the drive motor.  A 1/2&quot; drill simply does not have the torque or overall power required to turn a 12&quot; sanding disc.  The tool is going to bog and stall out anytime you try to use it.
Now, you see lots of Youtube videos where people make bench sanders out of 1/2&quot; drills... that's great, except those guys are using common 5&quot; sanding discs (you can tell by the 5 or 8 holes)*.  To drive your 12&quot; disc, you will need a much larger tool.  In fact, if you look at 12&quot; disc sanders, the nameplate on the motors, you'll see what is a good fit is for those discs - a big 1.25 horsepower 8A motor spinning at 1750 RPM.  (which isn't a good match for a 9&quot; angle grinder either, since it spins much faster9&quot; angle gr).
Anyway, the result is that torque will be so high that you're gonna need the 3/4&quot; shaft.
I recommend you use an adapter that will let you leave the 3/4&quot; shaft intact, so you can put it to use once you get a good sized motor.
Also, it's not going to work as a handheld tool.  Too small a motor and it won't sand.  Too big a motor and it'll kick way too hard.