https://docs.aws.amazon.com/dms/latest/userguide/CHAP_BestPractices.html
For a full load + CDC task, we recommend that you add secondary indexes before the CDC phase. Because AWS DMS uses logical replication, secondary indexes that support DML operations should be in-place to prevent full table scans.
That doesn't make sense, given that logical replication updates happen based on PK. (Or do they, if something on the source does UPDATE foo SET bar = 15 WHERE blarg = 12; and there's a secondary index on blarg?)