Typing an accessor for a Dictionary with defined keys

I am using typed dictionary to pass on a multitude of parameters to a few classes.

I have the dictionary typed like below, and an access for function that is typed with a separate type containing all the possible values. Is there a way to make this more efficient, i.e. refer to all the value types of the dictionary instead of duplicating them in a separate value type? (it is actually 18 different keys, so it is a bit messy)

typedef DbOptions as {
    :key1 as Numeric?,
    :key2 as String?
};

typedef DbOptionValue as Numeric or String or Null;

class Db {
    private var _options as DbOptions;
    public function getOption( option as Symbol ) as DbOptionValue {
        return _options[option] as DbOptionValue;
    }
}