I have had your larger trolley (BeachCart Folding (WZ1-BCF) now for 18 months.
I regularly take it to the beach. The walk to my usual spot is 4km. The trolley is usually loaded with 15kg-20kg of gear but feels like 5kg weight in hand while pulled behind. I use it on bush tracks that are holed and crossed with steps and tree roots, on sand hills and the beach. The trolley handles the torsion of weight while travel over rough terrain well and remains stable with loads piled up toward the handle. It has been one of the key finds in my fishing gear of the last 10 years. I could not walk long distance and fish with the comfort I now do without it. My fishing partner is persuaded and is now getting one also.