The 2mm of extra chamber length in a 9x21 won't hurt the accuracy much. It's less than the 1/8" (3mm) of extra with a 38 special in a 357 chamber, and those will shoot great. The killer for 9mm is the 358 cylinder throat and the 357 barrel. Those allow a 355 9mm bullet too much wiggle room. That's why 357/358 bullets work better. You have to tightly control the bullet from the case on out if you want it to shoot straight, and always go from larger to smaller going forward. A 356 throat and 355 barrel would fix it. Sadly, they aren't made like that.
I wonder if reaming out the throats as far as possible and pressing in interference fit inserts would work. That or face the cylinder off until it was just longer than normal 9mm loads, turn the barrel shoulder back to meet it and modify the yoke. Kind of like the short ACP cylinders and old custom PRC guns did. Less opportunity for the bullet to yawl.