When I researched what to use for my backup I found rsync.net. They have some nice features nobody else seems to support, like they support ZFS send/receive https://www.rsync.net/products/zfsintro.html

But in the end the price made me go with borgbase.com

I believe it just finds the user and not the post because the previous posts aren’t synced. So if you are the first user on your instance (eg lemmy.world) to look at that user, only the new posts that this user posts after you have subscribed are synced to the instance.