Our issue is that it takes 20hrs to index the full table. Hopefully we can add FK in multiple child partitions concurrently, otherwise doing it per partition offers no advantage from performance pov.
Need to test. Hopefully PG should not lock the referred table during the first build, stopping concurrent execution of other partitions.
In vanilla Postgresql (including RDS Postgresql) you can add the FK to each partition and then ALTER TABLE ONLY the parent table.
Don't know about Aurora, though