Just tried migration_lock: nil, pool_size: 1 and got a different error:
** (DBConnection.ConnectionError) could not checkout the connection owned by #PID<0.304.0>.
When using the sandbox, connections are shared, so this may imply another process is using
a connection. Reason: connection not available and request was dropped from queue after 2975ms.
This means requests are coming in and your connection pool cannot serve them fast enough.
Well, it’s progress 
So I can look into the underlying cause more closely
