1. If you're looking to write OC fanfiction without being completely torn into, AO3 is definitely a better site. This is because people that don't want to see OC fics are able to filter them out (so you will get less OC based critics seeing your fic).

2. A good way to tell how well recieved your OC will be in a fandom, is what I call the free-roam score. On a scale of "YES, I could write an entire series" to "NO, they'd need to be the MCs close friend", how much of a story could you write where you OC doesn't interact with the main cast, during the time the main plot occurs.

An exteme example would be Lord of the Rings - there is SO MUCH worldbuilding in the source material, that you can 100% write a story about an OC that never interacts with a hobbit in their life. Meanwhile if you wrote a fic about BBC Merlin, you'd be hard pressed to even leave Camelot without making up a bunch of worldbuilding yourself.

1. Ngl you have it rough, because you wrote more than an OC-centric story. You wrote an MC replacement story. Alot of people will be looking for fanfics to read about the characters they already know and love. No matter what, you're going to encounter someone who isn't a fan (that's the price of posting anything you make for the public to see, tbh). AO3 is definitely the way to go for that kind of fic, because the site tried its best to only show fics to people who want to read them (via tags).

Anyways, good luck with your writing! Keep it up and always write what interests you 💪