https://castboolits.gunloads.com/sho...g-BHN-increase
this 2020 thread is a pretty good read. It reinforces my understanding that water quenched hardening requires antimony in the alloy. Based on your volume of lead, I'd think getting it analyzed a cost effective must. Otherwise you may find yourself running in circles.
As for measuring BHN through powder coat, I can not imagine how you could ever get a consistent value.