Well physics of a boat in water, and a boat on trailer are completely different. The water is a shock absorber for the weight of the motor and boat , torque is lessened by water giving way to “twist” of the prop. All things being equal the difference is like putting your foot on a piece of wood. Floating it gives way until it hits bottom, on shore it bears the full weight of gravity. Loose or weak attach points affect transom stability in water, on trailer it is direct weight on the transom .
I am having a steel transom saver made by a local welder, I’ll post pics when he is done. Should solve the bending issue with aluminum.