We have something very similar, written in Java. Our extensions specify whether the migration is run pre-release or post-release, in order to facilitate no-down

We have something very similar, written in Java. Our extensions specify whether the migration is run pre-release or post-release, in order to facilitate no-downtime releases. The pre-release migration is responsible to not break the running code. The released code is responsible for detecting (if necessary), the pre- or post-release state and behaving appropriately. Sequence is to run pre-release migrations, deploy the release, then run post-release migrations. Sometimes days apart, in case things need to be stabilized. Testing environment runs only pre-release migrations, until some time before the release, then runs post. We also have a handful of keywords to run operations in ways that don't lock large, frequently-used tables, or do other environment- or application-specific operations in standard ways.