Burning 100 grains of powder in a short barrel is kind defeating the whole purpose of rounds like the 30 Nosler, 7RUM and the like. I agree with JM, if you want the speed you are targeting with that heavy a bullet you need the longer barrel. To perhaps clarify what JM said, throat erosion is strictly the result of the amount of powder burned, barrel length isn't usually a factor, except to the extent you may have to add a lot more powder to get the speed you want out of the short barrel.
I too am a lefty, I also like short barrels on my hunting rifles, and I llike light guns for packing in the hills. That said, I use a cartridge that works with 20" barrels in 5 LB guns. For my big boomers like the 7RUM, 338-378 and the like, I use 26-28" barrels on heavier guns and put up with the extra effort to pack them around.