The KRUX was uncomfortable to me. The XCurve was much better but never felt dialed. I thought the K4 was the best until I tried the ARK. I now have three ARK frames. I don’t know how it would do with the issues you spoke of but I truly think it’s the best frame I’ve ever worn.
Krux uses straight stays. ARK has some curve. (EXO K4 also has some curve). Which will work best for you, only you can say.
As well, a stock SG frame has a thinner lumbar pad than either Kifaru or EXO, which could be good or bad for a particular person, but they’ll send more foam to build it out if you need it.
I’ve had surgery on L5/S1. I tested every main pack (SG Evo, SG Crux, EXO K3, K4, Kifaru Duplex Lite, Kifaru ARK, MR Beartooth, and KUIU Pro). For my post surgery back the K3, K4, and new ARK have been by far the best comfort wise. My injury is a ways from yours but all the companies were great about returns as long as I left tags on. Did a few 5 miler 65 pound hikes with all daily changing my order until at the time K3 won for me. Since then, K4 and ARK are very close in comfort with nod going to the ARK. The K4 for me seems to pull back slightly compared to the ARK. I’d try to test a few and return them before your cc bill is due!