Lower hardness can be helpful if more toughness is needed. Going softer on D2 might gain a little toughness but also trades away a lot of the edge retention that makes it an attractive steel in the first place. Based on just a few tests I’ve seen from other QSP knives, they run their D2 around 58-59 HRC. Another potential issue could be the purity or consistency of the Chinese D2. We know from LTK’s XRF testing that the composition can vary on Chinese D2.
The German K110 should be better in that respect. The bad reputation on Chinese D2 has given Petrified Fish, Bestech, and a few other brands that usually use it a reason to start importing K110. Either way, the toughness is still only around 5ft/lb at 60HRC. I’d rather avoid these issues in the first place and use a steel like 14C28N. Aside from being much more stainless, 14C28N has roughly six times the toughness at similar hardness!
BTW, if you don’t mind messing up your knives then you can do a mustard test for D2. The steels used by unscrupulous companies to substitute for D2 are usually 3Cr13, 5Cr15, or 8Cr13Mov. All of those steels are notably more stainless. After thoroughly cleaning with isopropyl alcohol, you can put a blob of yellow mustard on the blade for an hour and see what happens. I recently tested this on satin finishes at around 68 degrees Fahrenheit . At just over an hour, Walmart’s Organic Yellow Mustard left a permanent gray mark in the shape of the blob on D2 but wiped away pretty cleanly from 8Cr13Mov.