And reddit -> lemmy migration script is going to be one of the easiest thing you can write. That is without the API limit and the sheer amount being the biggest problem...

What's there to federate about image cdn? If you don't want to use 3rd party service just self host. You just have to make compromises either way.

I'm not sure about the longevity but a lot of people use discord as CDN apparently. There's even an API for that.