…ith validate_assignment=True)

This is necessary to prevent a bug:
The isinstance(data, cls): return data shortcut in MigrationMixin.migrate_then_validate (a model_validator(mode="wrap")). Pydantic re-enters this validator during validate_assignment, passing the current instance as data. The shortcut returned it unchanged, silently discarding every field update. This broke all assignment — not just validation, but even plain cfg.count = 42 was a no-op.

The problem is that model_validator(mode="wrap") wraps the entire validation pipeline, including validate_assignment flow. Migration should only run during construction, not on every field assignment.
The clean solution is to switch from mode="wrap" to mode="before" — it transforms raw input data before validation and doesn't participate in validate_assignment at all.