Oh well, no. The database won't be decentralized. That just invites chaos.
Bookwyrm did that and now there are lots of duplicates in their library, which I definitely want to avoid. The things that will be federated are animes' forums (as communities in Lemmy for example), and those communities will have threads that will be either a forum thread or a review which others can comment on from any federated platform. Some other things might get federated in the future.
I decided to just scrape either MAL alone or multiple sources.
If you have any ideas on how the database can be decentralized while efficiently avoiding duplicates and spam, please do say it.