Link interesting Lemmy posts to discord and other communities to draw people organically. Don't spam links, that'll just get you muted or banned, but do sprinkle them about the internet in moderation. This is how I've managed to get people I know interested in Lemmy anyway.

Dedicated discussion posts for the weekly anime episode could help. Currently the manga scans have weekly posts but don’t really receive a lot of comments, as most people comment on the website or on the other site.

As someone else said, it's all about creating interesting posts first. I know that's the most difficult part, but the community needs to organically grow in quality of posts first (not that those that are already in here are bad, it's just that they don't create much engagement).

Not a Onepiece fan, but... get the Discord to plug Lemmy?

Almost all my TV fandoms that live on Reddit seem to be deteriorating and migrating to Discord. I think people from a Discord would post here just to escape that dreadful format.

Yes, I would enjoy those regular posts one expects from the reddit counterpart for instance.