how do i repost on instagram without losing anything?

If Add to story isn't showing under the paper plane, that's not a bug. It only appears when the account is public and hasn't switched resharing off. Private accounts can't be reposted that way at all.

paper plane icon under the post, then add to story. keeps their handle on it as a tap-through so nobody's wondering where it came from. what i'd stay away from is screenshotting it or downloading and re-uploading. it gets recompressed so it always ends up softer than the original, and the credit's gone. if you want it on your actual grid, just ask them — most people say yes if you tag.