I'm a little worried Lemmy may not be ready for prime time. I've already had a few times where my post to another instance just vanishes. But when I made an account on the other instance that worked fine. It seems like choosing the right instance is more important than I initially thought.

Is this because the admin of one of the instances blocked the other from posting? Does it federate by default or is it on an approved basis? I'd like to spin own instance as well but depending on how interop works between instances that also seems like a poor decision.

I'll keep looking but a more informative FAQ would be great

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.