sealed class 적용 시...

state는 freezed를 이용해서 구현하면된다는 말씀이신가요?

네. sealed 키워드만 넣어주세요

import 'package:freezed_annotation/freezed_annotation.dart';
import 'package:flutter/foundation.dart';

part '$NAME$.freezed.dart';

@freezed
sealed class $CAP_NAME$<T> with _$$$CAP_NAME$<T> {
  const factory $CAP_NAME$.success(T data) = Success;
  const factory $CAP_NAME$.error(String e) = Error;
}