I think I need to take the story down from AO3 when I do finish it & put it on Amazon, which is also a drawback, so I'm not sure about it.

On the other hand, the AO3 interface is MUCH friendlier to commenting than Tumblr, so there's def. less of a "IS ANYBODY OUT THERE?" effect when you post there. If someone is out there, they'll let you know! So there is no perfect solution.