Tried second brand of NiZn (UltraCell) and met with similar results as the PK Cell NiZn. So for me that closes the book on NiZn.
Just tried 8 of UltraLast LiFePo4 AA cells bought locally (https://www.batteriesplus.com/productdetails/ul14500sl=2p) and surprisingly they performed only slightly better than the NiZN and could only maintain 11.7v under load. Very disappointing.
I have some of the LiFePO4 cells you suggested on order, so hopefully they will get me the same results you got.