Distributed Availability Group Direct Seeding FAILED, failure_state SQL Error, failure_state 2
The AlwaysOn Professional blog has some general troubleshooting steps for direct seeding and also includes some details about trace flag 9567 to enable compression during seeding, but I didn't find any details about the SQL Error or Seeding Timeout.

We previously have had issues with large databases causing problems in availability groups, but this usually is resolved by applying the latest transaction logs from the primary against the replica.

In this case the databases were listed on the secondary availability group as recovering, so I tried applying the latest transaction log backups from the primary and then joining the database to the secondary availability group:

--Restore transaction logs from primary and stay in recovery mode. Multiple backup files may need to be restored from oldest to newest.
RESTORE LOG stackoverflow from disk = '\\Backups\SQL\_Trans\StackOverflow_AG\StackOverflow\StackOverflow_LOG_20170810_175400.trn' WITH NORECOVERY;

ALTER DATABASE stackoverflow SET HADR AVAILABILITY GROUP = [StackOverflow_RAG];
ALTER DATABASE stackoverflow SET HADR RESUME;


This worked for both of the failed databases and fixed the replication issues. Our reporting cluster now has all databases kept in sync from the primary availability group: