Yes, it’s super easy. Amazon has an email address for your kindle, all you need to do is email the epub or pdf and it shows up on the kindle automatically. This is the first guide I found on Google. https://www.howtogeek.com/798894/how-to-transfer-epub-to-kindle/
In the case of social network like this, bigger generally is better for the users. The thing that made Reddit great was that whatever your niece interest, there was a community of thousands of other interested people. There was so much information and advice on whatever obscure topic.
There’s a reason why there’s only around 10 really popular social networks and it’s certainly not that those platforms are any good. The network effect is important.
IOS is worse. You can’t even be sure that it’s saved, let alone where.