Wouldn't it be possible to use a boost converter with the PD charger to get that last .4 VDC? Also, I would think someone familiar with power supplies could tweak the voltage within but that might be pushing the components to an early demise.
AFAIK, the independent power supplies (Megawatt/Meanwell) are the easiest solution for achieving the higher voltages needed for 6V GC2 batteries.
I programmed my Morningstar solar controller to do the brain work. It must be doing good work as the SG levels are at baseline while in storage.
