The django docs state that the --keepdb flag should keep the test database from being destroyed after a test run, and that subsequent test runs should not recreate the test database if it exists already.
https://docs.djangoproject.com/en/5.1/topics/testing/overview/#the-test-database
For reference the POSTGRES django adapter does this.
https://github.com/django/django/blob/main/django/db/backends/postgresql/creation.py#L42-L45