first guess .. 10mm x 1 thread per mm pitch..
shimano uses that one, sansin may also..

if it breaks too often, consider a solid axle and nut fixing ,
the cones and washers should fit a solid axle..


Permanent cure , Phil Wood rear hub , freewheel type are fine .
(cassette are way up there in price)
I toured a couple years worth of loaded trips and they're still fine.
You will never break those axles.