Moved over to zip no problem, but I can’t find a way to export my comments/posts. I know I can access my profile via zip but will that be cached and available after the servers are shut down?

No, Lemmy has a memory hole problem right now. All the content you created is only on the servers online and federated at the time you created it. Once those servers go offline, for any reason, that content is gone

Incorrect, that content is mirrored to any of the instances that federated with Lemm.ee and will continue to be accessible; although if you interact with any of it after Lemm.ee is gone, that interaction (vote, comment) will only be visible on the instance on which you did it.

As far as I understood, the already posted/federated content is stored by each instance in its database. The pictures/media might be gone but the text should be copied around depends on (de)federation policies

Not quite right there. The comments exist on all the other federated servers as they’re sent out as part of the ActivityPub protocol. The problem is there will no longer be an easy way to aggregate them. Posts may disappear. But comments won’t.

If instances A B C are online and federated when you post, then A B C have copies of the data

Let's say A shuts down and you move to a new instance D

Only B and C have your old content. D has none of your data. There is no backfill mechanism

If in the future B and C go offline (eventually they will) then the data is gone. Maybe in a archive.org snapshot.

Even in the future if D E F G still are going strong and Lemmy is thriving all the content that only A B C saw is gone. It's a bitrot problem

Yes that's correct, new instances don't backfill old content. Only the servers online and federated at the time of posting will have the content.

If you migrate to a new instance that wasn't online or federated earlier it won't have your old content

There is no backfill mechanism. Only the servers online and federated at the time of posting will have the content. New servers will not. If you move to a new server it won't have your old content