We still don't know the origin of the rinnegan but if you ask me i'd say the juubi wasn't the origin of the rinnegan.
I don't think his body was stronger than his eyes here
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we see madara saying he'll improve the power of his eyes by becoming the juubi junchuruki and cast a powerful genjutsu on the world. It shows that becoming a junchuruki can improve the users eyes... Where am i going with this you may ask...? Simple, the sage eyes were improved when he became the juubi junchuruki... It's a possibility that there's another stage of the sharingan after the rinnegan or the sage acquired the rinnegan after becoming the juubi junchuruki which i think is unlikely because madara have acquired rinnegan without having the juubi.