i haven’t found any info for syncing accounts across instances (to prevent this loss) or if this is even possible at low level

Not possibly or very difficult from what I heard.

what happens with comments and pictures once a federated instance goes down? say, if i selfhost and i crash without backups, does my content and posts disappear? Everything is stored in a database, so crashes are no different than any other site. It comes back and its accessible again.

what kind of capacity planning would I need to selfhost say, a decade worth of reddit browsing? assuming I only care about my posts and what I save, would they be accessible for me?

You mean like host an instance for just your own account? The smallest instance for $5 a month should be enough.