Might Taiga be anywhere close to what you’re looking for? It’s a desktop client for MAL, Anilist or Kitsu so it does require at least one of them as a backend, but it also stores your list offline in an editable form. For example if you make edits without internet connection, you can sync them back later. You can’t add new shows offline since it needs to fetch the data from those services but you can edit watched counts and scores.