I'd be looking at less than 40lbs if going slow. Very commonly in our 4wd club trips we will drop tyres from 34-38 (road pressure) to 23-25lbs, and occasionally in sandy conditions to 12-15. Not recommended by tyre manufacturers of course but entirely practical and successful. With Ebike soft ride a premium over rugged ground - esp on older bodies. Experiment would find the practical minimum for retaining steering. And from a year with my earlier E bike, it had very good batt life/range from its larger 21 AH battery and softer tyre pressures would not have drained it too quickly.
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