My Wife has had both knee's replaced so the right step was important to us as well.
We had 3 criteria a candidate had to meet:
1. Strong & folds up compact
2.Sturdy & Relatively large platform (comes in handy especially at night, and will not tip easily)
3. needed adjustable legs ( to often the ground is not even, so this allows us to maintain a pretty level footprint)
It took us a while to locate something that met all lour criteria:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Admittedly they are more expensive than most, but fit all our requirements.
We also use it as the platform to stand on with the outdoor shower.
Side note: Larrie, Have to say I really like the storage step idea!