You say, "There is no compelling reason to not use a prop designed to fit on your 25mm shaft." I agree. But no one I've talked to yet (about 30 different shops) knows where to get metric prop nuts. A couple are looking into it. And I can't find any online. Do you have any resources? Also, the anode will have to mount on the prop nut as there is only about 1/2 inch of exposed shaft available between where the prop ended and the stern tube begins.
See pic at: