Using arguments in addConstraint() for validation
Options needs to be an array I think. You can access it on the constraint object in your validator, see ListStringItem::propertyDefinitions() and LengthValidator, which then uses it as $constraint->max.
Base fields are exactly that. Base fields. There is no bundle, they are the same for all bundles of an entity type (they can be overriden per-bundle, but then they become accessible in the bundle hook).
You shouldn't ask two different questions together :)