Agreed.Too bad you bought all those mags! A CSA Vz.58 would be an awesome upgrade from the AKM. You should be fine with any of the mentioned AKMs though. I have 4, a SAM7R, Draco-C SBR, SAR-1, Milled MAK-90, and the Vz.58 is the highest quality, lightest, most accurate of the bunch. Have fun picking!
I believe that 7.62x39 is right on the edge of acceptable use for long-stroke piston. Anything bigger and it's just too much moving mass. Most operating systems have their sweet spot, with the obvious example being straight blowback in lighter calibers. This might even be true of direct impingement. Dunno. I assume this is why short stroke pistons are fairly popular today, because it can be tailored to the caliber requirements without giving up too much in accuracy.