As you said, decentralization is key. Highly active human moderation is the only known solution to keep communities free and tolerant, and human mods have a relatively low limit as to what they can handle without making it a full time job (or burning it out)
The Lemmy network is still centralized enough that many smaller instances make the calculus that it's better to be federated with the large weakly moderated instances than to lose access to the many small communities on those instances.
But increased decentralization makes more granular defederation possible. A weakly moderated instance can simply be blocked.
I think we'll get there in time.
Block and report. We're all in this together!