I've been mulling over the different possibly processes to put splines on a shaft. I came up with a way to do it where no material is actually removed and it would actually make the metal in that area stronger.
Instead of cutting the splines out of the bar stock, why not do it like knurling or threads and squeeze them into shape? It would mean the metal would have to be annealed and also hot. Run it through a press that has the correct spline pattern on it by rolling it through 2 sets of teeth that act as gears as well as the die. Once the splines are made onto the shaft, then re-process the metal to the desired hardness and also do a surface hardening treatment, such as nitriding. If this can be done with something like 300M steel, then I think that is as close to bulletproof as can be expected with the stock diameter and spline count.