I have the same question. Using Dart native solutions and reducing code gen is definitely preferred but freezed isn't easy to replace

Since there are too many examples on the freezed README and the one at the top isn't a good use case to begin with (I like to keep my data models (DTOs) separate from entities, and DTOs are good enough with plain json_serializable), I'll provide an example from one of the projects I'm currently working on. It is still more verbose than it would usually be with freezed, however, I'm pretty fine with that. Also, it's worth noting that whenever I need a copyWith, I still use codegen with copy_with_extension. It has a nicer copyWith API and only handles that instead of a bunch of other stuff I don't necessarily need.

part of 'simply_browser_bloc.dart';

sealed class SimplyBrowserState with EquatableMixin {
  const SimplyBrowserState();
}

class SimplyBrowserInitial extends SimplyBrowserState {
  const SimplyBrowserInitial();

  @override
  List<Object?> get props => const [];
}

class SimplyBrowserLoading extends SimplyBrowserState {
  const SimplyBrowserLoading({this.loadedSimplies});

  final List<Simply>? loadedSimplies;

  @override
  List<Object?> get props => const [];
}

class SimplyBrowserFailed extends SimplyBrowserState {
  const SimplyBrowserFailed(this.failure);

  final ApiFailure failure;

  @override
  List<Object?> get props => [failure];
}

class SimplyBrowserLoaded extends SimplyBrowserState {
  const SimplyBrowserLoaded({
    required this.canLoadMore,
    required this.simplies,
  });

  final bool canLoadMore;
  final List<Simply> simplies;

  @override
  List<Object?> get props => [simplies];
}

And then using the sealed class itself becomes super-nice, like the following snippet (only wrapped in a function to state clearly where the variable is coming from):

  List<Simply>? getSimplies(SimplyBrowserState state) {
    return switch (state) {
      SimplyBrowserLoading(:final loadedSimplies) => loadedSimplies,
      SimplyBrowserLoaded(:final simplies) => simplies,
      _ => null,
    };
  }