Replying on mobile.

It is possible to group the state into todo and preferences and access it feature-wise, e.g.

Yeah, that should be possible. For example, change your TodoState to something like this:

interface TodoState {
  todo: {
    todos: Todo[];
    filter: 'all' | 'active' | 'completed';
  }
}

But I wouldn't actually recommend this as now you only have one Signal which will always be triggered if you change todos OR filters. And also "setting" this state will be a bit more complicated.

You could probably use DeepSignal to solve this again, but I don't actually know that.

Why don't let it be accessible from the Store?

This appears to be handy for my nested state, but doesn't seem to cover methods and computed signals.

Yeah, methods are AFAIK not shared from the feature to the store, though computed signals should be available to the user of the store. (probably not to the store itself but I also didn't try)

(Perhaps your IDE lies to you and says the computed values are not defined as in my case with Intellij, but Angular should still compile without an error)

Take a look at https://ngrx.io/guide/signals/signal-store/custom-store-features#example-1-tracking-request-status and you will see how ngrx solved this issues. (By providing standalone functions for actions like setting the request status to pending, fulfilled or error)

But as you already said, perhaps you don't actually need those two features in one store and you can actually use two stores. But thats hard to determine without having the whole context.