@jerry I wonder if a dual-tier blocklist that propagates might be a solution.

For the "trusted" tier, an admin can subscribe to a list where blocks occur automatically when forwarded from a trusted instance. It's still an instance block, but it'd block-and-notify, which the other admin could monitor from time to time.

For the "other" tier, blocks would be advertised to an admin via feed, but not automatically implemented. In that way, the instance would require human intervention to evaluate and/or block. If an instance gives good recommendations for blocks, this might be data for adding their instance to the trusted list.

Vetting a trusted list would be per-instance, but the effort would be far lower. If you trust instance X, you set it and forget it. For new instances, interactions between admins as well as mod posture would govern whether you'd choose to trust their instance blocks or just get notifications. Even "other" instance block notifications could lower effort by bringing the instance to others' attention.