Laravel groupBy return array of items instead of using date for key

I'm using the Laravel groupBy method to format my data and group them into periods, however, I'd like to change how I return my results. Rather than using the created_at as the key for an array item, I don't want there to be a key, and instead, just move that key into the returned result instead...

The following code will return:

"results": [
    {
        "2021-03-04": {
            "event_category": "category",
            "event_action": "action",
            "event_count": "544",
            "period_from": "2021-03-04 00:00:00",
            "period_to": "2021-03-04 10:30:02",
            "created_at": "2021-03-04 10:30:06"
        },
        "2021-03-03": {
            "event_category": "category",
            "event_action": "action",
            "event_count": "5337",
            "period_from": "2021-03-03 00:00:00",
            "period_to": "2021-03-03 23:57:03",
            "created_at": "2021-03-03 23:57:08"
        }
    }
]

Here's how that looks

$res = DB::table('my_table')
         ->select('event_category', 'event_action', 'event_count', 'period_from', 'period_to', 'created_at')
         ->where([["event_category", "category"], ["event_action", "action"], ["period_from", ">=", "2021-03-01"], ["period_to", "<=", "2021-03-04 22:00:00"]])
         ->orderBy('created_at, 'desc')
         ->get();

$results = $res->groupBy(function ($item, $key) use ($findable) {
  $date = Carbon::parse($item->created_at);
  return $date->format($findable['groupByFormat']);
});

$results = $results->map(function ($item, $key) {
  return $item[0];
});

$res = $results;

I'd like to return...

"results": [
    [
        "event_category": "category",
        "event_action": "action",
        "event_count": "544",
        "period_from": "2021-03-04 00:00:00",
        "period_to": "2021-03-04 10:30:02",
        "created_at": "2021-03-04 10:30:06",
        "new_created_at": '2021-03-04'
    ],
    [
        "event_category": "category",
        "event_action": "action",
        "event_count": "5337",
        "period_from": "2021-03-03 00:00:00",
        "period_to": "2021-03-03 23:57:03",
        "created_at": "2021-03-03 23:57:08",
        "new_created_at": '2021-03-03'
    ]
]

I've tried removing my initial map, but not sure how to achieve such format whilst remaining the grouping