Since you are using createAsyncThunk you can do something like this:

  dispatch(fetchUser())
  .unwrap()
  .then((user) => {
    // do anything you want with user, or don't, also dispatch actions
    dispatch(fetchSubs());
  })
  .catch((e) => {
    // error in case of rejection inside createAsyncThunk second argument
    console.log(e);
  });

Explanation

Let's say const thunk = fetchUser() so basically dispatch(fetchUser()) is the same as dispatch(thunk).

Redux's dispatch function returns whatever its argument (the action) returns.

So in this case, dispatch(thunk) returns whatever thunk returns.

thunk, based on how createAsyncThunk works, returns a promise that either resolves to fulfilled action, or the rejected action. (those actions that you receive in extra reducers).

This is how you can access those actions:

dispatch(thunk).then(fullfilledAction=>...).catch(rejectedAction=>...`

RTK library also provides a method called unwrap. Instead of those action objects I explained above, it lets you use the returned value from the 2nd argument of createAsyncThunk.

export const fetchUser = createAsyncThunk("user/fetchUser", async () => {
const user = await Reddit.getUser().then(val => {
    return val;
});

return user; // unwrap lets you use this instead of action objects.
})

Try this with pure react and redux hooks

...

const state = useStore(yourStore) //use your method to read state 
const dispatch = useDispatch();
const checkValue = () => {
   const stateMatch = window.location.href.match(/state=([^&]*)/);
    const codeMatch = window.location.href.match(/code=([^&]*)/);

    if ((stateMatch && codeMatch) || localStorage.getItem("access_token")) {
        return true;
    }
    return false;
}

useEffect(() => {
    if(checkValue())
        dispatch(fetchUser());
    }
}, []);

useEffect(() => {
    if(checkValue() && state.authState)
        dispatch(fetchSubs());
    }
}, [state.authState]);


...