Good to know there's some tolerance for adding primary keys after partitioning something this big. Could you have aborted if it wasn't going to complete within your window?
In both cases, success or failure, before swapping a second time we needed to copy forward all the rows being inserted into the placeholder table.
Other disaster mitigations are capturing a dump of rows for the relevant partitions being modified with pg_dump. And having physical database backups with snapshots enabled and available if things really go wrong.