“Our source database is about 400GB in size.”
GOV.UK Notify moved 400GB and 1.3 billion rows from PostgreSQL 11 to 15 with AWS DMS, ten days of replication and a DNS record with a 1-second TTL. Cutover took 11 seconds against a 5-minute target. The whole job exists because the government is retiring its own PaaS. Taxpayers funded the platform, then funded the engineering to escape it.