What I've seen work well, for multiple migrations, (at an admittedly smaller scale) is using shadow writes/reads and a source-of-truth toggle. The API layer ma

What I've seen work well, for multiple migrations, (at an admittedly smaller scale) is using shadow writes/reads and a source-of-truth toggle. The API layer made requests to both the old DB and a new DB that had been populated during a small window of scheduled downtime. We spent a couple weeks/months running checks in production that the old DB and new DB were returning identical results, though still returning the old DB's results as source of truth. Eventually, we flipped the source of truth to the new DB, and some time later decomissioned the old DB.