How simplify code for Custom Type in objectbox?

Question

I am trying to optimize the code definition/implementation of a Custom Type on an Objectbox Entity using Flutter and Dart.

To declare a custom type in an entity I proceed in this way.

@Entity()
class Car {
  /// ObjectBox 64-bit integer ID property, mandatory.
  /// Add `@Id()` annotation if its name isn't "id" (case insensitive).
  int id = 0;

  @Transient()
  CarTypeEnum carType =
      CarTypeEnum.sportive;
  
  int get dbCarType => carType.managedIndex;
  
  set dbCarType(int managedIndex) {
    carType = CarTypeEnumExtension.from(managedIndex);
  }
}

My problem is that I have a lot of custom fields and the code becomes very hard to maintain and read.

It's possible create a class using Objecbox attribute (like @Transient) in a subclass to factorise the previous code ?

@Entity()
class Car {
  /// ObjectBox 64-bit integer ID property, mandatory.
  /// Add `@Id()` annotation if its name isn't "id" (case insensitive).
  int id = 0;

  
  CustomType<CarType> carType = CustomType<CarType>(CarTypeEnum.semicolon);
}

I search how implement CustomType class