AFAIK Ecto 3 locks tables and uses two connections to the db during migrations. Maybe you can use some combination of :migration_lock option and the number of connections pool_size: 1 to work around your problem.
I couldn’t make your example work since it’s missing a dependency:
Unchecked dependencies for environment dev:
* sqlitex (deps/sqlitex)
the dependency is not available
** (Mix) Can't continue due to errors on dependencies
