Yea, knights would be the next best choice for trading offensive stats for defensive. Which piece depends entirely on your preference for how much extra defense you want. If you want just a little bit more toughness, then you swap for the pieces that have lower stats (eg. boots or gloves). If you want more toughness, then swap for the higher stat pieces (eg. chest). Most people I've seen seem to swap their head, chest, and legs when they attempt to mix offensive and defensive stats, but that choice is pretty much arbitrarily based rather than on any math or anything. "Berserker armor with Knight's jewelry" also seems to be a popular (albeit arbitrary) choice.
However, I would highly reconsider even wanting more "tankiness." As it is, this build and its traits are already on the tanky side and the end result is almost inevitably longer kill times with no difference in times downed, which is probably 1 or close to 0 already. Is there any specific reason why you want more tankiness? If it's because you just want to "feel" more tanky or get downed less, you probably won't notice much difference throwing in a few pieces of gear, and like I said there may not be any quantitative improvements in survivability either. Mathematically though, you will be quantitatively doing less damage.