Hmm... aluminum is more susceptible to fatigue, but for the same weight, is a lot stiffer, because beam stiffness is (IIRC?) a cube ^^3 function of thickness, so even at 1/3 the stiffness modulus of steel, is 27/3 = 9 times as stiff. But steel is stiffer and handles fatigue better. I don't think they are fatiguing due to column buckling, but rather, lateral bending, the weight on the rack pushing left and right while rocking the bike. Gonna think on this more...

If you don't need all the holes, leave out the ones you don't need. The strut broke, not at the bolt hole, but at the open hole below it, at the natural stress concentration that the hole provided. Thinking more...

Go either thick high-strength aluminum, at least as thick as originally, or stainless steel only a little less thick. You need the stiffness that the thickness provides, unless you use a different section shape than just flat, for more stiffness. Thinking more...

I think I might take a piece of stainless steel hydraulic tubing, hammer or vice it flat at both ends, drill one hole at each end for mounting. Cylindrical closed section for the bulk of length will provide better stiffness. However, there may be stress concentration at transition from round to flat. Thinking more...