Ill keep it short, sites like MAL or anilist REALLY need to let you review anime and manga per story/arc instead of a whole. or is there such a superior site already available somewhere?
On AniDB you can vote per season (temporary vs permanent vote, and review rating), as well as per episode, release group. You can also rate characters and voice actors.
Example of Season Vote Graph; you can see per episode on the bottom right:
Filler episodes are also marked as such so you can skip them without watching etc.
It differentiates between rating and average, and gives you a predicted rating for you when you made some ratings.
I get what you’re saying, but that sounds incredibly complicated and almost impossible to do for the thousands of manga and anime they have. Unlike episodes, seasons, chapters and volumes, arcs mostly aren’t clearly defined beforehand and sometimes can span over multiple seasons or volumes with a bunch of sub arcs inbetween. They’d have to actively keep up with all anime and manga in existence for them to know the arcs. They don’t need to do that for seasons, episodes, chapters and volumes. If you want to review arcs, the best option would probably to create a forum entry on the series.