I wouldn't worry about stressing the frame over time issue one bit. I'd trust that the makers have done the appropriate engineering to make that point moot. Many folks seem to get fixated on the fact that there is no fatigue limit for aluminum while not giving appropriate consideration to these two things.
1) Engineers who design aluminum bike frames are well aware of fatigue limit of aluminum and take it into account when making their designs.
2) If stressed beyond a certain point, steel frames also have a fatigue limit. They only escape the fatigue limit issue if they are never stressed beyond that point.
There is a stress test posted online where conventional lightweight bikes that use different materials were put through tens of thousands of stress cycles. They did this until either the frames failed or the cycles hit their limit of 200,000. And while certainly not an exhaustive test, the conclusion was that for lightweight bike frames from reputable makers, the steel frames were generally the least robust - even though the other frames weighed substantially less. This wasn't so much a matter of material, but was almost surely more a matter of the engineering and construction was used to make the various frames.
https://www.sheldonbrown.com/rinard/fra ... e_test.htm
The KMX specs suggest something possibly similar or related in that their aluminum frame trike has a maximum load of 300 lbs while the steel bike frames all max out at 209 lbs. That suggests to me that the aluminum frame is probably more "over-engineered" (and stiffer) than the steel frames. The quality of the engineering and construction are more important than the material chosen when it comes to durability. And how can a lay person judge the engineering and construction quality well? So people tend to judge based on what they can clearly see and that's the material that is used ... which really doesn't tell them anything important regarding actual real world durability.
I'd choose the material based on other factors as previously mentioned. Things like whether you intend to modify the frame, frame flex, weight, frame dimensions, and so forth.